I haven't seen any US P-40Ns so far that weren't state-side trainers. I suppose it is possible some did (as some went "backwards" to P-39s instead of going to P-47s or P-51s, late in the war). However, all the MTO ones I recall reading about switched from P-40s to P-47s while still flying the F/L/K models.
As for the Merlins: I know they were in use. However, when you compare a P-40F and a P-40E, the FTH change doesn't really make much of a difference because of the dogleg power curve. I never said it wasn't wide-spread, just that if you're looking for a decent lineup it would be redundant, as the F can stand in for the E or the E can stand in for the F. There's no real improvement.
I think it might be nice to split up skins though (long tail vs short, merlin vs allison, etc). I don't really count it as vital, myself.
As I said previously the MTO squadrons were Merlin P40s
As for the combat N models. I really think you are confusing it with another version as the N was everywhere. Since a picture is worth a thousand words, here's 8000 words on Combat N models I could from just the books on my shelves post another 30,000 words at least of combat Ns from all over the map

49th FG as they arrived, June 43 PTO

49th FG as they finished up, September 44. No this is not a hack, but a frontline bird just before they gave them up. PTO

RAAF P40Ns PTO

15th FG P40N in Coral camo. PTO

P40N with DT and rocket tubes. CBI

Line up of 51st FG P40Ns CBI

80th FG P40Ns CBI. The skinners would have a field day with the N model

Heavily loaded Aussie N flying with the RAF in the MTO
