Thanks for an interesting write-up, Good Boy.
Your mom doesn't sound like a person who would get those intimate and sexy items of clothing on a whim. I think the fact that she had them is pretty clear evidence she had an active sex life in her appartment. I guess the young colleague is a likely candidate, but as you said, she could have any number of different lovers. You need more information to reach a conclusion.
She also sounds like a woman who keeps her guard up. So I suppose it would be very difficult to get her to loosen up a bit. Maybe you could talk with your aunt and persuade her to help.
I think I missed out on the time frame a bit. She just had her second child, and the living in her own appartment is some time ago?