Could you check this over

For me, the simplest explanation for the Greek form is just as an athematic -mn̥t-. Greek regularly loses final -t, so in the nominative the result would be -mn̥t > -mat > -ma or -mn̥t > -mn̥ > -ma (both orders are possible and give the same result). In the other forms, the -t- was not final, so it was retained. I see no reason to involve suppletion, unless I am missing something.

No, you've probably not missed anything. I'm probably overthinking this. I definitely have seen the theory that -menta > -mentum; though, now I cannot seem to find it now. Would you say, however, that it is fair to categorize -mentum and -μᾰ as abstract t-stem derivatives of *-mn̥?

I suppose so, although I don't know what abstract t-stems are, exactly.

Search for "t-stem" it on this page. They would also give forms such as:

• *-tuHts: Latin -tus, Celtic *-tūts, Gothic -𐌳𐌿𐌸𐍃 (-duþs).
• *-teh₂ts: Latin -tās, Greek -της (-tēs) and Sanskrit -ताति (-tāti).