RM ANOVA vs. ANCOVA question

#1
Hi stats friends! I'm trying to decide whether to use an RM ANOVA vs. an ANCOVA to test a question and I could use your help.

Participants were randomized into 3 groups (categorical between-subjects IV: Factor A), and two continuous variables (let's call them B & M) were measured at 3 time points (continuous within-subjects variables), but I also have a trait measure (continuous) for M that was measured before the experimental manipulation.

I want to test whether M moderates the relationship between A & B. So my question: is this best tested by using my trait measure of M, and using a RM two-factor ANOVA, or an RM ANCOVA with M as the covariate? I suspect that M is related to B (my DV), though, so this would violate ANCOVA assumptions, correct?

If I wanted to use my momentary measure of M, is there a way to test whether M moderates the relationship between A and B? Help! Clearly I need to meet with my advisor ASAP...