You started writing really weird conflated stuff. What do fees have to do with transaction syntax? ... The amount of fees doesn't change the syntax, so doesn't require change of the version.
Sorry, I don't understand your objections.
There are no "meta-rules" that specify what the validity rules can be. They are not limited to "syntax", whatever that means. Any computable predicate on bit strings could in principle be a validity rule, as long as it does not completely break the system.
Right now there are no validiy rules that refer to fees. The minimum fee, like the Pirate Code, "is more what you'd call 'guideline' than actual rule"; each miner decides whether to require it (or even to require more than it). But the minimum
could be made into a validity rule. the difference woudl be that each miner would not only impose it on his blocks, but also reject blocks solved by other miners that contain transactions that pay less than that fee.
The version field should be used to clearly describe syntax rules governing the transaction format.
As I wrote, this cannot be guaranteed. If a fork (rule change) was executed to fix a bug or prevent an attack, the miners cannot continue to use the old rules for transactions that have the old version tag; that would negate the purpose of the fork. They must reject such transactions.
So, it is not safe to retain signed but unconfirmed transactions without broadcasting them.