What stops miners from sending their tokens somewhere, then hard forking from a previous block where they didn’t send them?

This has a clear answer in PoW, but not in your scheme

Easy, you put the transaction in the previous block. So the miner can only mine a block if they have already made the expenditure.

What if I create a whole new chain? Will the transactions at the very beginning of that chain have to be added to the big, old chain? How far back do the common ancestors of two chains have to be for your auto-merging to happen? What happens if two histories conflict? Then in Version Control parlance, we have a "merge conflict".

What stops me from doing a hard fork to roll back both blocks?

If you spend more tokens than the tokens that were spent mining the last two blocks you can erase the transactions in those blocks, but isn't that also the case with PoW?

You aren’t following. The hard fork lets me double spend the tokens from those blocks.

Indeed, I'm not following your argument.

Stop asking people to do your homework.

No. Did you not read the comment you were responding to?

Yes, I read it. As far as I can see, it talks about 'staking', so it doesn't address what happens when the tokens and 'expended' as opposed to 'staked'.

You have two competing chains: Chain 1 and chain 2. On chain 1, you spend a token on a good. On chain 2, you spend no token. You wait for chain 1 to win the race. You then stake on chain 2 to get your money back. If chain 2 then overtakes chain 1, you've executed a double-spend attack.

An equivalent attack wouldn't work on a PoW chain. If you do the equivalent of "staking" on chain 2, then you're computing hashes, which is costing real-life resources. In the PoS case, without slashing, staking on chain 2 is free. In fact, this is the rational move to make every time you spend a token; stake on competing chains to get your token back.

Thanks for explaining (I'm a different someone)

There was a PoS mechanism that makes people who cheat lose all their coins? I wonder if it is relevant here

... Aha, that's "slashing" -- the other members in the network would look at the two chains, and notice that you were misbehaving, and add transactions that remove parts of your coins? (They'd add to both chains? Or just the winning one?)