A soft fork isn't required to use a different version number. Some numbers cannot be used, but, for the most part, miners can set whatever block version number they want. This is what allows them to use the version number to signal soft fork readiness and also allows them (unfortunately) to use asicboost.

Yes, the scriptWitness is malleable.

While you could malleate a transaction to be larger, I'm not sure why you would. You can't change the fee that is paid by that transaction, so your malleated one would have a lower fee rate than the original, so it would most likely be rejected in favor of the original. That also means that the malleated transaction is less likely to show up in a block than the original transaction. Additionally, such malleation would make the transaction non-standard, so most nodes would discard it regardless of whether they had seen the original.

AFAIK it's the opposite.


Most Bitcoin full nodes (whether it's run by miner, exchange or regular user) would reject such block because there's no valid redeem script or the UTXO is invalid.

Let's start with something obvious,
1. Maximum Bitcoin script is 40000 weight (or 10000 bytes before SegWit activiation)
2. Non-turing complete Opcode