Both uses of the past form are possible. Some grammarians speak of the past form as "remote", because it "removes" the verb to a distance which may be either temporal or social.
Hi, Maria, I'm calling because I wanted to take you out for dinner.
Here you use the past form to make want less demanding. The remoteness is social.
Sorry I couldn't track you down last night. I wanted to take you out to dinner.
Here the primary sense is clearly temporal remoteness. You could, however, work in a degree of social remoteness by also employing the progressive construction, which in this case would have no implication of imperfectivity - it would just be a further distancing device.
Sorry I couldn't track you down last night. I was wanting to take you out to dinner.
Note that this last construction could also be used in the first example:
Hi, Maria, I'm calling because I was wanting to take you out for dinner.
In this particular instance, however, the progressive construction would probably not be used because it clashes with the progressive calling in the main clause.
- Grammarians give this the Latin name horror aequi, "the
widespread (and presumably universal) tendency to avoid the use of
formally (near-)identical and (near-)adjacent grammatical elements or
structures" (Rohdenburg).
You'd be more likely to use it in a context without a prior progressive:
Are you by any chance free tonight? I was wanting to take you out for dinner.
To express the "see-what-you're-missing" sense in English you would employ a construction with a stronger sense of intention. The past would be necessary in this case, rather than optional, because you would be speaking of a prior intention which no longer holds:
I was going to take you out for dinner, but I've changed my mind.
No, I don't think it's impolite. For instance:
"Excuse me, can you help me please?"
"Sure - what's up?" - seems like a polite enough way to ask what the problem is, although certainly very informal.
But as a response to someone asking you to fill out a questionnaire, it doesn't seem to make much sense. It seems like your response just needed to be yes or no at this point.
Best Answer
It's fine as a response to an apology. However you should be careful not to use it if the accident is your fault (even if they apologize first) since, "It's all right," implies that you forgive the other person.
Other responses:
Also, there's an Australian expression, "No worries!" which I like to use even though I'm American.
Edit. Please review the comments, as there are many regional differences in the nuance of these expressions.