Phyrebrat, I agree with MHP that the scenario you’ve outlined isn’t credible in legal terms, though the number of people who would realise that is likely to be very small, so you’d probably get away with it, especially if you were vague about exactly what happened each time. If you want the legalities to be authentic, though, you need to do some tweaking. My thoughts on the ideas as they stand at present:
1 The church acts as tenant-in-chief of the land. That means the land has been given to the Benedictines (not to the church) by the King, which in 1176 is Henry II. He definitely established some religious houses in France after Becket’s death by way of atonement (or the appearance of such), so that would kind of fit in, but the disadvantage of this is that it’s not until 1387 that tenants-in-chief can alienate land, so there’s no way your dishonest Abbot can dispose of the entirety of the land in the 1340s, which is what you presumably want to happen. Plus if it’s a royal grant, I’m pretty sure the projected abbey would be expected to be something grand, and the land allotment would be sizeable (don’t forget that the grant would involve not only the place where the abbey is to be built, but also sufficient lands around it/in the area so it’s self-funding through farming) so the chances of it deteriorating to a leper hospital are lessened. Since there’s no advantage in the grant coming from the King, and possible disadvantages, I’d suggest making the grantor someone less exalted.
2 Shielde conspires with the corrupt Abbot/trading post. I have to confess the “trading post” idea confuses me a good deal, as it’s not something I’ve ever heard of in England. The usual course of events would have local people gravitating to the abbey in order to benefit from its presence – selling to the monks/receiving charity – and a settlement would gradually establish itself around the abbey buildings, which would grow into a town. There’s no way an abbey in that situation would ever be converted to a leper hospital which were always kept well away from settlements for obvious reasons. If this were a purely contemplative Order that might make more sense, as they would discourage people settling nearby, or perhaps a daughter house of an established monastery, which might remain more isolated. But I’m still not sure what you mean by “trading post” – if the site is convenient for trade, ie at the confluence of trade routes or rivers – then it’s more than likely the site would already have started to develop into a town in the 150 years since the abbey’s founding, even if not before. I’m also not at all convinced by the trading guild idea you have, but that’s another issue.
3 Shielde frames the church for crimes. The church as a body can’t commit crimes, of course, not in a legal sense at that time anyway, and the abbot personally would have benefit of clergy, a rule which meant no cleric could be tried in or punished by a secular court (save for a few exceptions such as high treason), and in any event, as MHP says, the land is not vested in the abbot personally, but in the Order, so even if the abbot were arraigned and executed, the land would remain in the possession of the Order who could simply appoint a new abbot straightaway.
4 Under the development of escheat, the 7 traders become feoffees. There would be no escheat in these circumstances, but if it were to happen, the land would revert to the overlord, ie in this instance the King, who would then hand it out to someone else willing to pay enough money for it. No feoffees would be appointed in that situation, as there's no Trust involved, and trustees certainly wouldn’t be a pack of local oiks with no money or name. In any event, I wonder if you’ve missed a trick here. If you’re planning a takeover by Shielde in the 1340s, why not push it back a few years until after 1350 by which time the Black Death has decimated (used in the colloquial not technical sense) the local population, so the small town which has grown up around the abbey/leper hospital is entirely lost, and Shielde has less to worry about when he comes to take the land for himself.
5 Shielde does away with the others. If trustees die, others should be appointed in their place, though this is more plausible than the other bits. By the way, what makes this place so attractive that Shielde wants it and is willing to kill for it? And where does he get the money from to convert the place to his home and maintain it thereafter?
I don’t know how wedded you are to the plot as it stands, and especially the trading post/guild thing, but for what it’s worth I had an idea which might hold legal water, or at least sounds more inherently plausible. Have the church steal the land in a two-fold way, but push back that theft to 1098 and the first crusade. A crusader owns a big swathe of land which has been granted to him and his heirs, and to protect his wife and baby son he establishes a trust whereby the abbot of the nearby abbey is appointed feoffee of the entire estate. The abbot knows of the pagan stones (which the crusader has refused to remove?) so he decides to establish a daughter house there to sanctify the site. He therefore appropriates that bit of land at least (the less he steals, the easier it is to hang onto it – stealing the whole creates greater noise and sympathy for the wife and son) and evicts the people in actual physical possession, pagan or otherwise. The crusader never returns, his son dies five days short of his 21st birthday, so can’t inherit from his father and bring the trust to an end, but he already has a legitimate son of his own (special dispensation allowing the marriage), so there’s now a dispute as to whether that son can inherit when he gets to 21. Lots of legal actions, rumbling over the years. The family eventually gets the rest of the land back, but the Order manages by shady means to keep the once-pagan land (though why it would want to is another question). The family fortunes wane, the Black Death kills 80% of the local population, but Shielde finds the heir who is now on his uppers. He forges a few documents with the help of the crooked abbot or some other associate (if Shielde is only a miller or farmer without assets of his own, he will need access to a lawyer or educated man to deal with the legalities), takes the land back from the Order, then kills both the heir and the abbot/associate after having the heir make a Will in his favour. (Hmmm. Would need to check when it's possible to leave real property in Wills.)
Mind, I also still like using the dissolution to prise the place out of the Order's hands.
Not to pour cold water on moving the boundary stones idea, which is fine for getting agricultural land or woodland, though it requires Shielde to have sufficient property alongside it to make it credible, but it requires something of a leap of faith that no one -- even after the Black Death -- thinks it odd that suddenly a complex of Benedictine buildngs are miraculously within his boundary markers.
Finally, I've had another thought about the Sheedthorpe Grange name. "Thorpe" as name and suffix comes from a word meaning small village/hamlet, which doesn't seem appropriate here when there's the leper hospital, and places incorporating it are mostly situated in the area of the Danelaw, with a few exceptions in Surrey for some reason. It's not something in use in the Wiltshire/Hampshire area as far as I'm aware.
