Is it OK to add a question mark to show inflection?

I think you are referring to cases such as:

A: I'm so coming with you later!
B: Err... No?

In informal writing such as chat, it's perfectly acceptable, and other similar "stylistic" choices are fine.

In formal writing it should be absolutely avoided, since to express the same function there are other ways to achieve the same result in a better form.

All three of your examples are questions; they're just not worded precisely as such.

There is no problem at all in doing that (everyone does it, so it's good English), as long as the reader understands that the person speaking was asking a question -- so putting an interrogative point at the end clarifies that the person is asking for a reply, not simply making an exclamation.

If you are writing, you should not put question marks on non-questions.

However, none of your examples are non-questions. All those question marks are appropriate.

The questions themselves are fragments, or else improperly executed, but that is forgivable in speech. As long as such questions remain quotations, you shouldn't have a problem grammatically.

The question mark, in formal writing, is exactly that; it indicates that the previous statement was a question. It is not strictly a mark of the "high rising terminal inflection" normally used to indicate a question, and so it should not be used to indicate this in cases where the inflection of spoken word does not necessarily indicate a question. Most use of the inflection in cases other than a question came about in the past 20-30 years with the popularity of "valley girl" speech, which is a localized, however popular, form of speech, and not "standard" English.

However, there is an exception. Within quotation marks in a narrative, indicating a character's speech, it is generally acceptable to intentionally misspell, abbreviate, and "mis-punctuate" statements in order to convey the tone, accent, or cadence of a character's words if that is important to the narrative, or for comic relief. For instance, it has become acceptable to indicate slow, pointed, very clearly-enunciated speech using periods, such as (from The Host) "Who. Is. The. Seeker." In such cases, using question marks to indicate a character's speech inflection may be allowed for this illustrative purposes. If you do this, it has to be readily apparent that that's what you are trying to convey.