Learn English – Why isn’t ‘oranger’ correct even though it follows being a one- or two-syllable word for adding the comparative inflection

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.

Your reasoning (You cannot treat "both of us" as a list of two alternatives because 'both of' explicitly means they are not alternatives) is correct.

There is no syntactical explanation. "One of (the) both of us" is as grammatically sound as "Colourless green ideas sleep furiously". It is the semantic content of the words that makes both them unacceptable.