All that conversation proves is that the private contractors that answer the phones at NMC don’t know what they are talking about. Nothing they say is binding on the USCG. That’s not news to most of us.
Your national near coastal Master of Towing authorizes you to sail on near coastal Tugs up to 200 tons without full STCW endorsements. STCW does not apply to US vessels under 200 tons in US or Canadian waters. The large majority of US coastal Tugs are under 200 tons.
Your national license as Master of Towing near coastal authorizes service on Inland vessels (but not on Western Rivers) of any size.
Your national license as Master of Towing would authorize service on near coastal water on the vessels up to 300 tons, IF you also had full STCW. But you don’t.
There are only a small number of US Tugs between 200 tons and 300 tons. So you are not missing much there.
It’s worth having a 500 ton national license from a perception of better employability perspective (even though that perception is really a misperception), even if you don’t have the STCW.
It’s also much better to have to National license and exam behind you, and only need the classes to add STCW.