It depends on how you pronounce it. If you pronounce the middle vowel as in man, you should divide it ba-nan-a. If you pronounce the middle syllable as in spa, you should divide it ba-na-na. The first is the predominant American pronunciation. The second is the predominant British pronunciation. (And thus, the first hyphenation is the standard American one and the second is the standard British one.)
The reason the pronunciation makes a difference is the rule: never break a stressed syllable after a short vowel (i.e., /æ/, /ɛ/ /ɪ/, /ɒ/, /ʌ/, /ʊ/). There are, of course, exceptions to this rule if all the other hyphenations are worse (i.e., ra-tion).
According to Swan in Pratical English Usage (p114) the two-syllable adjectives whose comparative form is most likely to be formed with -er are those that end with an unstressed vowel; e.g. narrow, simple, clever, subtle, etc. from your list above. Swan goes on to state:
With many two-syllable adjectives (e.g. polite, common) -er/-est and
more/most are both possible. With others (including adjectives ending
in -ing, -ed, -ful, and -less), only more/most is possible. In
general, the structure with more/most is becoming more common. To find
out the normal comparative and superlative for a particular
two-syllable adjective, check in a good dictionary.
It is interesting that Swan himself uses more common and not commoner in his explanation above, and this seems the better choice in a formal written context. So if you are looking for guidelines for your advanced students I would recommend:
- Learn the common two-syllable adjectives ending with an unstressed
vowel that can be compared with -er.
- For the rest use more. I suspect that native speakers are much more
likely (likelier?) to consider an -er usage problematic than a
more usage. For example, more polite or even more clever will probably sound less ungrammatical than pleasanter or
tranquil(l)er.
If your students really would like to know word-by-word if the -er comparative is possible, they will need to consult a good dictionary. Swan recommends the Oxford Advanced Learner's Dictionary, the Cambridge Advanced Learner's Dictionary, the Longman Dictionary of Contemporary English, the MacMillan English Dictionary and the Collins Cobuild English Dictionary.
The Collins, for example, shows pleasanter but more tranquil as the comparative forms.
Best Answer
The phonological process observed here is anaptyxis, which is the insertion of a vowel. In English, the most common vowel thus inserted is the schwa, as seen in your two examples.