Spock lived in this one to his olden days and saw the unification of two tribes, Vulcan and Romulans. And the most curious thing about this is, is that the Vulcans are more strict and harsh while Romulans has softened up over the years.
I don't think that's quite correct. They state early on that Spock had died, but later on indicate they're not sure about what happened to him, which suggests that it appeared he died in the red matter explosion but they could not confirm what happened, which is in keeping with what was presented in the Kelvin Timeline.
So anyway, what about the C-plot in which Saru has seen fit to promote Ensign Tilly to acting First Officer?
It's not quite as crazy as it first appears.
Discovery is down (as per episode 302) to 89 active crew. There's quite a few cadets and other ensigns on board, and most of the other Lieutenants, Lt. Commanders and Commanders mentioned over the show's run seem to have vanished or been killed. We know that around 50 crewmen left the ship to remain in the 23rd Century. I strongly suspect all of the higher-ranking officers with families will have remained behind.
Canonically then, at the start of Season 3, Tilly is only definitely outranked by 11 people: Lt. (jg) Owosekun, Linus & Bryce; Lt. Detmer, Nilsson & Rhys; Lt. Commander Culber and Stamets; Commander Burnham and Reno; and Captain Saru.
You eliminate Saru and Burnham for obvious reasons.
The Best of Both Worlds established that non-bridge officers and heads of departments cannot be XOs (Riker doesn't even consider LaForge or Dr. Crusher for the position), so that eliminates Culber and Stamets, probably Reno as well. Detmer is having medical issues, so you take her out of commission. Owosekun's position at Conn may also remove her from contention (in the
TOS-era, helmsmen don't seem to have been considered as an XO-compatible position even when you're on the command track; Sulu went straight from Conn to Captain of the
Excelsior without taking up an XO position).
That leaves four people ahead of Tilly. I'm going to assume that Linus (dinosaur guy), who has been used mainly for comic relief so far, is not seen as command material. Nilsson (blonde bridge officer) seems to have been Acting XO in episode 302 and the start of 303, so I assume decided she didn't like the job or Saru decided she wasn't suitable for the position in a longer term. So you're down to two: Rhys (Tactical) and Bryce (Communications). You can combine Tactical with XO (Chekov in
The Wrath of Khan) and I don't see why Communications couldn't as well.
However, there role of XO in the 23rd Century
Trek is not the more formal position it is by the
TNG era. There's no such thing as a dedicated "First Officer" role pre-
TNG, it's always combined with another position. One of the responsibilities is almost HR-like, acting as a liaison between the crew and captain (this is something Spock actually struggled with a few times before getting a handle on it). The position requires the trust of the crew and a strong rapport with the captain, and we've seen Tilly have both in the past. Rhys and Bryce could do the job, no doubt, but on that interpersonal role it's clear that you'd go with Tilly.
Having an Ensign in the position is unusual - which could be solved by promoting her to Lt. (jg), which she deserves by now anyway - but again it is precedented that a junior officer can command superior ones. LaForge as a Lieutenant in
The Arsenal of Freedom comes to mind. Riker also considers making Worf XO in
The Best of Both Worlds when he'd only just been promoted to full Lieutenant, putting him ahead of Data, LaForge and Dr. Crusher. In an episode of
DS9 O'Brien also says it's not an uncommon practice, and in
Valiant emergency battlefield commissions are even given to Cadets, putting them in CO and XO positions (admittedly that was an extreme circumstance, but
Discovery's not far off that at the moment).
Also, it is like Riker leaving the Enterprise and Picard appointing Ensign Wesley Crusher.
Wesley was only made an Acting Ensign and by definition the most junior and least-experienced person on a ship with 1,012 personnel on board (an estimated 600-800 of which were active Starfleet duty personnel). Tilly is, as far as can be told, one of the dozen most senior people on the ship with two years of experience (including in warfare) under her belt by this point.