If x....could happen

I analyze these logically as two possibilities, where one is conditional on the other's becoming true. If possibility A becomes true, then possibility B becomes active. If possibility A does not become true, we need not consider B.

There is a possibility A that they see us; only if that becomes true (they see us) does the possibility B of their killing us become active. Otherwise (they don't see us) we are safe.

There is a chance A that my mom will let me; only if that becomes true (she lets me) is there a possibility B that I attend the party. Otherwise (she does not let me) I cannot attend.

Hence, none of your statements guarantee anything about the outcomes of the various possibilities.

Yes, those statements convey possibility, not guarantee or certainty.

My understanding is that the word "could" is properly the past tense of "can" and therefore means "was able to," but in common usage it has acquired another meaning akin to that of the conditional mood (with the meaning "would be able to" or "might"). It can sometimes be helpful to avoid the latter usage when writing English because the meaning is a little less precise.