Can transaction validation really be free?
There is currently no simple way to pay for transaction validation in order to mitigate those risks. I think there should be.


This statement is incorrect. Transactions include a fee which the miner that finds the block will get. Therefore it's in the miner's interest to include transactions and thus validate them.

Once transactions are in blocks, it's in everyone else's interest to validate for themselves and they don't run any risk doing that. (They'd run a risk NOT doing that.)

There is currently no real problem of transactions getting stuck because "it's too risky to validate". There are a few miners that mine empty blocks, possibly for this reason, but more likely because of lazyness and misunderstanding. And that will become even less of a problem every time the block-rewards halve.