David,
Regarding: "Magazine drops free, and once fully inserted it can still move upward a little bit more (magazine base plate doesn't touch grip frame)."
The up and down play of the magazine you describe is normal. Most feeding problems stem from a malfunctioning magazine. As noted above, get a new Mec Gar magazine and make sure that the hold open area is clean and that the hold open operates freely. Your original magazine can likely be restored by GT.
KFS
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