Is the question mark misused in affirmative sentences?

While the sentence is not technically a question, the use of a question mark indicates that the speaker is inviting a response from others.

In that respect, the question mark conveys the speaker's meaning in a way that could not otherwise be indicated in writing without the use of additional words.

So I'd say that this use of the question mark is correct when recording informal speech like this - but formally, it should really be reserved for actual questions.

I now have a bit more time to fix bugs etc but I'm open to offers if anyone wants to help maintain this module with me?

This sentence/question is different than a statement and different than the "up-talking" that happens when people occasionally (or all the #$! time) raise the pitch of their sentences and phrases. If the person said

I now have a bit more time to fix bugs etc but I'm open to offers if anyone wants to help maintain this module with me.

they are merely stating that they have time and are open to offers for help.

By raising the pitch at then end (spoken) or adding the question mark (written) they are implying a question:

Would anyone like to offer to help me?

I hear this type of usage commonly.

We will proceed with the next item on the agenda, unless there are any questions? no? Ok, moving along...

This is something you see very often in spoken language.

As we talk, we are making a syntactic structure. It's common for syntactic structures to change, or be re-analyzed, as we go along. So in this case, "if anyone wants to help maintain this module with me?" is reanalyzed, on the fly, as "Does anyone want to help maintain this module with me?"

In careful writing, since readers can go back and check what you said previously, this sort of on the fly reanalysis is discouraged.

Steve Melnikoff's is the best analysis, but can be taken further.

I think with the 'quick text parsing' that is becoming a part of our language pragmatics given the 'texting'/'information overload' age, punctuation marks in written materials are changing their purpose. As Melnikoff point out, in the example you use the question mark is inviting a response. The reader can visually 'skim' right through e.g., an email, to get at what his interlocutor wants him to respond to. The question mark is in effect basically an abbreviation with respect to what would have been written twenty years ago, when more verbiage would need to be required to have the request not come off in writing as rude.

English in the twenty-first century (in a particular context, at any rate).

Another example which I don't think quite fits any of the explanations above occurs in social media where the question mark is used at the end of declarative statements intended as accusations or harsh judgements. The context shows the statement to be actually declarative, though it could be considered an interrogative in other contexts. However, the poster doesn't want to come across as socially abrasive or intends to leave room for backing off of the judgement in case there is a backlash of disagreement or reverse judgement. For example:

"She is a narcissist?" "A guy that overweight shouldn't be going to McDonald's?" "Those so-called 'statues' are just toys for overgrown children?"

In this case it is a fundamentally disingenuous usage, because the poster clearly has a strong negative opinion about someone or some group, such that they want to express it, but don't feel either confident enough in themselves or the view to post it as a genuine declaration. Another way to put it would be as an unconvincing attempt at avoiding social liability.

Just for the sake of anyone reading, I found this thread when I googled about this issue because Microsoft Word grammar checker tried to tell me that I should put a question mark at the end of this sentence: 'If you can do them in advance of this deadline then so much the better.'

I completely disagree with such a usage in written English.

If a question is implied, the question mark should also be implied (i.e., not employed)!