Learn English – Polite alternative for “Is there any reason why we still haven’t done this yet.?”

There's nothing at all wrong with the "I want to make sure that" part of your sentence; the part that I find a bit obnoxious from a politeness aspect is the passive construction "it would be appreciated." You'd be much better off expressing your appreciation directly rather than simply allowing for the possibility that some unknown object somewhere will be gratified:

Further, I'd like to make sure that the dates of the courses will not conflict with my other appointments, so I'd be grateful if you could supply more details about the arrangements and what their costs would be.

(There are also a few other minor things that I've tweaked, particularly the placement of "that" in the first phrase, and the use of the singular pronoun "it" to refer back to the plural "arrangements".)

There's a bit of a disconnect in that you've specified that you care only about the dates but then you go on to explicitly request cost information as well; but if you've mentioned the cost aspect prior to this sentence, it's not a problem.

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.