Q: iCloud and iMessage deletion questions. Help me retain some privacy!

So, the situation is, is that I have a very snoopy mother who makes me give my password to her when it comes to iCloud and stuff. Before, I was on her iCloud for some years and never made my own. When I was on her iCloud, she could use Find My iPhone to track me and go through my messages and iMessages. Once i learned that she was looking through my messages, I decided to change to a new Apple ID for a sense of "liberation". In only 3 or 4 days, she's found out that I've switched and wants my information. I'm ok with handing it over, but I want to know how can I protect my privacy when it comes to my messages. The only two devices of mine that are connected to my apple ID/iCloud are my Macbook Air and my iPhone 6s.

So my questions are: Can she even access my messages from just knowing my new apple ID and password or does she need more information to get to them? If not, Is there any way I could possibly delete the messages and message threads on any iCloud synced device so that whenever she signs in from here on out, that she wouldn't be able to access those messages I've deleted.?

She's not going to be on my iCloud for more than a few minutes at a time, I'm also wondering if since she's going to be signing out and signing in multiple times, will the past messages delete from her device once she signs in for the second, third, and beyond time?