> What are you trying to show? You seem to agree that the English spelling of /nt/ is "nt", the English spelling of /ŋk/ is "nk", and the English spelling of /mp/ is "mp". There is no possibility of "np", "nb", or "nm".

Consider "inpainting", "unbiased", and (as suggested earlier) "government", each of which is a synchronically transparent /n/ across a morpheme boundary, yet a cursory survey of recorded English speech suggests that it's pretty common for these tongue gesture associated with /n/ to be absent—infamously, the second syllable of the last routinely loses its coda altogether. This occurs across a transparent morpheme boundary, even with affixes productive in the modern language, even in learned usage.

English does have a lot more wrenches to throw in this, like producing nuclear nasals in a range of situations and not always assimilating across prosodic word boundaries—heck, it probably goes both ways in an utterance like "in my main menu". Words spelled "mp" are reliably [mp] in the modern language, but it's not a simple case as "mp" spelling /mp/ read [mp] and "np" spelling /np/ read [np]; English phonotactics also coerces the nasal in /np/ to a bilabial realization.

> because a bilabial stop doesn't use the tongue and so [n] is easily coarticulated

That doesn't sound quite right—this assimilation surely wouldn't be nearly as globally prevalent as it actually is if that were true.

Try it. While you'd think from the descriptions that a bilabial stop shouldn't care where the tongue goes, I think you'll find it quite challenging to coarticulate [n] with [b]—tongue positioning at lower teeth is pretty obligatory—and much easier to sequence them or produce [mb].

Clearly you can see the unnaturalness of lack of assimilation to call the attempt to do so "special effort"! So of course, the typical anglophone is not going to try to realize [n.p], they'll just see the <np> and read [mp] because that's what they would with any other internal /np/.

> So again - what would justify representing the Japanese sound as "n" regardless of context in languages where, unlike in Japanese, the distinction between "n" and "m" is meaningful?

Now, this gets to an entirely different issue: the purpose of the transcription. You seem convinced that the main goal of romanization is to provide a pronunciation guide for anglophones. But in the context of discussing a niche musical genre on the internet, that's not necessarily a high priority in the first place; you might care more about, say, searchability: we're looking for https://en.wikipedia.org/wiki/Denpa, not https://www.worldbank.org/en/programs/debt-toolkit/dempa.

And in a wider context, the principal users of romaji Japanese aren't anglophones; they're Japanese-speakers who for some or other reason need need to coerce Japanese text into an ~ASCII-subset representation, targeting primarily computer systems with that sort of limitation (most common case being keyboards via IME, hold that thought) and secondarily other people who can read Japanese; and naturally they make the distinctions Japanese makes and largely don't make the distinctions Japanese doesn't make. So unless backed by a marketing department, they tend to produce n (or nn as needed) for ん, because they have a tenuous grasp on how anglos spell [mp] in the first place and でmぱ is garbage that their IME won't convert into the right word, so why type that?

(This is also why pinyin can be the way it is, yet their IMEs have routinely have modes to ignore s-sh/n-ng/n-l distinctions.)