Ok, so it worked! I edited the animations like you suggested, just had to, besides key framing the root track to move, also edit the pelvis Y track so it would stay in place (it was the bone that was making the character move forward, instead of the root), and it works now:
motion matching in animBP also works with these 3 sequences, idle, walk, and run.
however I still have to figure out why the retarget transposed the root movement in the original animation to the pelvis bone.