As for a 't' trapped between /n/ and a vowel, I've heard it pronounced in three different ways:
Maybe the formal, standard way is to fully pronounce the /t/ sound:
Another way is to omit the /t/ altogether:
Yet another way is to pronounce it as a flap t (represented here by the letter τ):
These are how the Longman Dictionary on my computer articulates these words; romantic with a clear /t/, Atlantic without a /t/, and countable with a flap t (shown as /τ/ above), while all these words have a /t/ in their phonemic transcriptions.
So what's going on? Are there any rules as to when you should follow each pattern?