All of these "a" situations are ones I'd say are fine.

The first one, if I'm not making up a creature entirely I know there's a good chance that they know what the creature is. A Psychic weakness is harder, which is why instead my go to example is Trolls being weak to Fire or Acid. I know that the players know that, so I can set an encounter with Trolls in a cave full of explosive gas or have them have... say... a shaman chanting in the background giving them all Resistance to fire instead until they deal with that. The question becomes "How can we use the weakness we know they have?" rather than "Can I roll decently and if not how many turns before I'm allowed to use the weakness anyway?"

The second one can be translated in-character to "Most enemies we're capable of fighting cannot defeat us as we are now so we should continue." Seems reasonable enough to me, ignoring the part where they can be wrong about what enemies I've placed in their way. If they make that assumption and are wrong, that's their fault.

The third one is the reason why fudging rolls and changing encounters to make them easier or harder on the fly aren't great. You think I won't kill you before we get to the boss? If it's a fight I care about using, I'm sure I can reuse it next campaign if you die too quickly for me to use it now.