I've looked out my old degree notes as we had to do quite a bit on pre-C20th land law to understand the 1925 LPA. It might help if I regurgitate a load of it, so you've got some background detail. Sorry, this is going to be a long post.
Following the Norman Conquest all land in England was owned by the Crown alone (and technically still is, of course). However, ownership of an estate in land (that is, the right to use it) was granted by the king to his tenants-in-chief eg on condition they provided so many men-at-arms and horses when called upon (that obligation was called knights’ service). The church was no different from the barons – any land held by the church was still held from the King but on terms of spiritual service ie to pray and say mass.
The tenants-in-chief could grant a subordinate fief of their estate ownership, ie in effect sublet the use of (part of) the land. (NB at this time there’s no concept of leasehold as we think of it today so "sublet" isn’t accurate as a term, it just easy to use as an expression.) That new under-tenant could then “sublet” in his turn, all the way down to the man in actual occupation, called the tenant in demesne. In each instance the original terms of tenure would have been to provide men or materiel or to perform certain spiritual or lay duties (eg the tenure of socage involved having to provide agricultural services such as so many days ploughing per year). However, in the C12th and C13th these obligations attached to the land were increasingly commuted to money payments by the tenant.
This "subletting" was called subinfeudation, and in theory there was no limit to the number of steps it comprised, and as a result ownership of the estate in the land could get complicated with dozens of “sublets”. In 1290 the statute Quia Emptores was passed which prevented that. Thereafter any tenant below a tenant-in-chief could no longer grant a subordinate fief while retaining his place in the chain linking the King to the tenant in demesne; instead, if he wanted to sell the land, he had to sell all his rights in it and drop out of the chain entirely. eg the King grants the use of 10,000 acres of Cheshire to Earl Roger, who grants the use of 1,000 acres of those acres to Sir Geoffrey, who sells 100 acres to Farmer Jollice. Pre-1290, the obligations go all the way up, from FJ to SG, to ER, to the King. Post-1290, though, SG is removed from the chain and FJ owes the use of his 100 acres and any lingering obligations attached to the land direct to Roger, who has had to consent to the sale. (The requirement for the overlord’s consent was eliminated in due course, and in 1387 the tenants-in-chief could alienate their land, too.)
Initially the use of the land was granted for life, so at a tenant’s death the estate reverted to the overlord who could then grant it to another vassal, though in practice it usually went to the tenant’s heir, again for his life. In time, though, the land was granted to the prospective tenant “and his heirs”. That presented no problems when subinfeudation was allowed, as the heir simply stepped into his father’s shoes viz-a-viz the sub-tenants under him. After 1290, though, the heirs got antsy, because their rights had been lost eg Sir Geoffrey’s son Hugh no longer has the rights to the 100 acres sold to Jollice. Sons brought court cases trying to get back the land from the new possessors, but with no success and the concept of owning an estate in land in fee simple was born.
However, some land was granted in fee tail, ie to the prospective tenant “and the heirs of his body”. The courts tried to fudge that to allow for a tenant to pass the land to someone other than his descendants, but a 1285 statute, De Donis, put a stop to that and as a result mucked up the position of those who bought land post 1290. Thereafter the heirs of the body still possessed the land even if it had been sold eg in that event Hugh, Sir Geoffrey’s son, would own the 100 acres, not Jollice, no matter that Jollice had paid for it years before Geoffrey died. This meant the land was effectively inalienable and in due course no one would attempt to buy land if it were entailed in this way, but there must have been many people caught out like Jollice thinking they had bought land legitimately, only for the courts to decide against them. My notes don't say if they received any compensation for the loss eg the return of the purchase money.
In addition there were other tenures, the most interesting of which are: estate per autre vie (eg a man might grant land to his prospective son-in-law but on his daughter’s life, so at her death it reverted to him and the son-in-law couldn’t sell it or pass it to his children); estate by curtesy (if the estate had devolved to a married woman, on her death her widower was entitled to a life interest in the land as long as there were heirs of the marriage); estate of dower (when a married man died seised of land, his widow had a life interest in one-third of it, provided there were heirs to take the other two-thirds).
Because of certain legal issues arising out of the more complex tenures the concept of the Trust arose (called Use at first) whereby the land was held by trustees (then called feoffees). This was given a boost at the time of the Crusades when a crusader would grant the land to non-crusading friends/associates to the use of his wife until he returned or his son achieved the age of 21, intending to give his family protection during his absence/at his death. However, this wasn’t foolproof as the law stated that the feoffees were the legal owners pending the crusader's return etc and neither the grantor nor the intended beneficiaries had any rights in the meantime so they could be chucked off the land – unconscionable but legal. The Lord Chancellor as keeper of the nation’s conscience could remedy the common law’s defects and after the complaints became too numerous to ignore there were interventions which stopped short of attacking the legality of the situation but tried to ameliorate its effects eg by locking up the feoffees until they did the right thing by the wife and children.
As a side issue, which might help provoke ideas, over the centuries there was a growth of incidences of tenure, whereby payments had to be made on eg the death or marriage of a tenant, and the development of escheat where tenure was ended eg if a tenant had no heirs or he was sentenced to death after criminal conviction. At the same time a body of custom arose around so-called unfree tenure (where someone was allowed use of the land but on the overlord’s whim, not under specific legal obligations) which gradually allowed the at-whim tenants similar rights to free tenants eg allowing their heirs to inherit the use of the land on which they worked. By the end of the 15th century long established custom had the force of law in the courts, but that custom varied from county to county, even parish to parish.
OK, you might think none of that is relevant as the soon-to-be Grange** is owned by the church, but it's not quite that simple. The church isn't a single person like the crown, or a monolith that owns all and any land occupied by clerics and monks – land would be granted to specific people/communities. I've not checked but I suspect the grants would be to eg the Dean and Chapter of a cathedral for instance, or to the specific order of monks/nuns. But unless that grant was from the King direct, there's going to be an intervening lay person granting the land to the Benedictines at some point. (NB Apparently, since Franciscan monks couldn’t own property, land granted to them was always held in trust for their use, so perhaps changing the order might help!) That lay person's status and the terms of the original grant to him and before 1290 to his overlords would therefore be relevant in assessing whether the monks actually do own the land no matter what they think.
Even if the monks can be legally dispossessed by an overlord, quite how Shielde could parlay that into getting possession and legal ownership himself, I'm not sure. Destroying legal documents and/or forging new ones is always a possibility, of course, though costly, and outright murder of anyone who knows different is always a good thing. I'll carry on looking through my notes and see if I can find anything about adverse possession (squatters' rights) in the Middle Ages, but that's unlikely to be of help in this particular situation since it requires sole possession of the land without acknowledging anyone else's title, so if Shielde has paid anything to the monks to use the land it wouldn't work.
An alternative might be to leave Shielde's involvement until the 1540s and the dissolution, when he could buy it from the Crown. Deception and malfeasance could still come into play, eg if he lies about the monks and their behaviour and/or cheats someone else out of the purchase.
** Have you checked usage? I'm not convinced it would be used as a formal name quite so early.
EDIT: just re-read your opening post. Strictly, the church couldn't have "stolen" the land from the pagans in 1178, as the pagans could only legally have held it under feudal terms (ie under the subinfeudation I talked about above) which seems to me unlikely. The church could, of course, be the new legal owners and have forcibly dispossessed the pagans who were in actual physical but not-lawful occupation of the site.
