1. What if miners start sabotaging block creation after 2140? Would introducing some coins that haven't moved since January 2009 help?
What would be the incentive for miners to start sabotaging block creation? Mining honestly would still net them transaction fees. Mining dishonestly just for the sake of it earns them nothing unless they control 51% of the network, which is incredibly unlikely.
Further, if you start unlocking unmoved coins, you create a
huge incentive to mine dishonestly. If the total block reward is (for example) 0.5 BTC from fees, and you release 50 BTC, then there is a massive incentive for every individual miner to try to reorganize dozens of blocks to try to claim the 100x reward for themselves. This problem compounds the more additional coins you release.
Wouldn't it be better to change the consensus regarding UTXOs made with old algorithms when transitioning to new ones?
No. I think it is far preferable for some old coins to be stolen and reenter circulation by people who have sufficiently powerful quantum computers, than it would be for developers to overturn one of the key pillars of bitcoin and start deciding what happens to coins they don't control.