I’ve been teaching the same in my Pali classes and I’m glad to see the research you’ve done. One example I always gave was that sa.m would sound like the French word sans.
To me also the sandhis of the niggahita (like ta.m ca into ta~nca, and such - sorry don’t have pali keyboard here) clarify that it was originally a nasalization of the vowel. For this results naturally from nasalizing the vowel.
Also it being in the original manuscripts more like a diacritic does not really suggest a consonant perhaps. I’m no expert on this though.
If I recall correctly, I found this video helpful, although it’s on Sanskrit.
I don’t have quick access to the Warder book), but I think Warder is relying (correctly) on the Samantapasadika on that point re closed mouth (avivaṭena mukhena):
Not that “anunāsika” means much here. It could equally be referring to a sound involving both the oral and nasal cavity (anu+nāsika), rather than a pervasive nasalisation of the vowel.
We must say, becoming a bhikkhunī comes close to do as well as one can in this world, but your high-school teacher, taking probably a worldly perspective, would perhaps disagree …