Usage of 'if' and 'if not' to mean 'and perhaps even/also'

I think, in this construction, if is essentially equivalent to although. One can substitute although for if, salva veritate, in the examples given, and in all the examples I can think of.

I might add that this particular if construction is an "aside", often set off by dashes or parentheses in writing, or otherwise almost always by commas. All of these punctuation marks indicate that the tone and volume are low, with flat intonation through the phrase, which is a mark of a part of an utterance that's not meant to be parsed as the main predication.

"Aside" is a stage direction for lines to be delivered not to the other players but to the audience.

A co-worker just encountered this same issue with these sentences:

"I feel good if not very mobile."

The interpretation really depends on which word the negation is attached to. I assumed [if] [not very mobile]. My co-worker assumed [if not] [very mobile] as in "I feel good if not great". In the first, [if] means 'although' or 'albeit' because [not] is negating [very mobile]. In the second, [if not] go together and mean as above 'and perhaps even' or 'almost'.

A test might be to insert the word 'quite' after [if not]. If the sentence makes no sense, [if] likely means 'although'and the negation goes with the following words. If it does make sense [if not] probably means 'and perhaps even'.