syllabic N in Japanese (idea)

The basic structural unit for building Japanese words is the syllable. Traditionally, Japanese has 51 syllables (although some are considered obsolete), 50 of which follow the pattern consonant + vowel. (Note that the consonant can be null and the vowel can be short or long.)

But there is one exception: the syllabic nasal N (/n~/ in IPA/ASCII), a sound which does not exist in English. The syllabic N is pronounced as its own syllable (ie. its length is one mora), even if followed by a vowel. The difference in pronunciation is subtle but important in some contexts. For romanized Japanese, if the syllabic N is non-final and is not followed by a consonant, an apostrophe is postfixed, like so:

kinen -- "memorial", no syllabic N: ki/ne/n, 3 mora
kin'en -- "no smoking", syllabic N: ki/n/e/n, 4 mora
kinnen -- "recent years", syllabic N: ki/n/ne/n, 4 mora

In addition, the syllabic N undergoes phonemic changes in some contexts according to the following rules:

"n'" is pronounced "m" if before P, B, M (eg. sanmai, three sheets)
"n'" assimilates the following consonant and is pronounced "ng" (IPA/ASCII /N/, English sing) if before K, G (eg. ningen, human)

The first rule also presents a problem for transcription: should the pronounced M be recorded instead of the written N? Most sources for the layman do so, but scholarly writings (and word processors) usually follow the original Japanese.