I agree with you on her lack of presence on the boards.
Honestly, I feel that MR is still learning how to play the game and developing some athleticism. I have feeling that she dominated in HS by virtue of being 6-6 and having a good touch around the rim. But, she came in woefully under developed in terms of strength, quickness, and overall endurance. Now her foot issues obviously had a lot to do with those limitations but on the other side, they did not appear to be limitations until she had to start matching up at the D1 level.
She has come a long way (it is odd to say but the so-called #1 recruit was a major project player). MR is still behind the curve in regard to her lateral quickness and core strength. Accordingly, she still struggles to post up against reasonable post because she can't beat them to the spot or muscle her way in. Those same limitations carry over to her problems in consistently boxing out. Yes, she can work on being more aggressive and upping her effort but she is having to work much harder than post players who possess more natural quickness. And given that she has to play major minutes, there are going to be some inevitable spells where she does pace her effort.
[I make this observation as someone who has said much of you have said and shared your frustration. Then I realized, Mercedes is probably doing all she can physically do at this point in her development process]
I noticed against Vandy, she was actually jumping a bit better and that is a great sign because that means she is developing whatever fast twitch muscles she has and is learning to make more explosive plays. If she keeps working and were to stay another season, she is likely to become a very dominant post player. [I have a feeling she will be WNBA bound but as a LV fan, I really hope to be wrong on that prediction]