Ton, the issue is that Nemesor's ability triggers at the start of the turn, and we have this snippet in the rules for reserves:
Moving On From Reserves, page 136 wrote:A unit cannot charge, or use any abilities or special rules that must be used at the start of the turn, in the turn it arrives from Reserve.
Adaptive Tactics, Necron Codex wrote:If Nemesor Zahndrekh is your Warlord, you may select a different Warlord Trait for him (no D6 roll is necessary) at the start of each friendly turn after the first – this replaces his existing Warlord Trait.
So, the answer to (1) and (2) is
yes - as Ton pointed out, we have no restriction on using abilities in reserve beyond being unable to draw line of sight or measure anything.
For (3), we invoke the Sequencing rule (page 17) as we have two things trying to resolve at once - Adaptive Tactics and arriving from reserves, that both happen at the start of the turn:
Sequencing wrote:While playing Warhammer 40,000, you’ll occasionally find that two or more rules are to be resolved at the same time – normally ‘at the start of the Movement phase’ or similar. When this happens, and the wording is not explicit as to which rule is resolved first, then the player whose turn it is chooses the order.
Now, if you choose to arrive first, then Adaptive Tactics cannot be used, but if you choose to use
AT first, then you're right, if you pass the reserve roll you end up in a situation where you have broken a rule in retrospect.
Despite writing all that, I've just realized I don't actually know which side to take... My
HIWPI would probably be roll for reserves first, so that you can't break the rule.