First and third seem fine, I’m concerned that “The modification replaces text that only exists once in the ruleset.” unbolts some trapdoors by being potentially much less explicit than the other two.

Even if we safely assume that bad faith scams (where someone subtly misquotes a dynastic rule in order to amend core) are ruled out by fair play, there’s still room for accidental misquotes, or (more likely) trouble with the queue. If somebody’s proposal has a casual “replace X with Y”, where X is a phrase that appears in both core and dynastic, it will be written and read as if this is automatically and safely restricted to dynastic. But if a proposal further up the queue removes that phrase from the dynastic rules, we’re suddenly amending core.

I think the second is safe. It could be shored up or additionally removed though.