Could you check this over

In both cases I believe it was reanalysis.

• For -mentum, there are examples like armentum, whose earlier form (armenta) is 1st declension. This would then have been reanalyzed as the neuter plural.
• Similarly, I suspect the that -μᾰ was suppletive (*-mn̥, *-mn̥th₂) and the t-stem crept back into all the singular forms except the nominative singular.

This may just be idle rambling from having looked at these too long, but it seems plausible to me.

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).