First question: I've read different answers to your question on the forum but, in my experience, the more time that passes, the more levels the sim will drop. She'll definitely not retain her current level.
(Is it dynasty-illegal for you to make a copy of your dynasty save file and use the copy to test this?)
In response to your second question:
After she leaves the science career, she can no longer use the invention constructor or the machine for creating serums. If she's invented a SimRay, serums, cloning machine, etc., she and her household can still use those.
Once she reaches Level 10, she can upgrade the wormhole generator so the whole household can travel together to Sixam for those cash-crop exotic plants and other rare collectibles. I realize you are thinking about how many satisfaction points this spare can generate.
Can't she go back and do Computer Whiz and Renaissance Sim later, after she maxes Scientist in 5 more levels?
You can always list up the requirements for those two requirements so you're finishing some of them while you're in the Science career.
Just my two cents. Sorry I couldn't be more accurate about your first question.