It will be "Could I have seen her?" in interrogative form

"Could + have + past participle" is used to past facts. In your example of "I could have seen her" is saying that you didn't see her but in the end you could have seen her. "Could + have" is used to things you wish that happen in the future, for example, "I could have a dog", is something you might do, is something you wish in the future. I hope I had answer your question :)

Yes you can. Subject + Could + have + p.p shows a possibility in past which could affect the present.