Learn English – Past tense means more polite

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.

Not sure if this is exactly what you want to understand. I didn't read through the other posts. Solely going by the examples you gave:

There is still a condition that needs to be met in both instances.

First example, it is uncertain that the guy whose fingerprints they found is the actual burglar. He hasn't been formally convicted of the crime. Therefore, he is still only a suspect until further investigation and prosecution.

"Why did he steal the money?" Means that he did steal it but they want to know the reason.

"Why would he have stolen the money?" Means they want to know the reason this person (provided he is the culprit) wanted to steal it (possibly to use this as evidence in a prosecution.)

Second example, I can't pinpoint because I don't recall this passage but it seems that it's possible someone else investigated and B didn't get the opportunity or something to this same effect. B somehow didn't meet a certain condition in order to actually investigate but you can't make the claim that he actually investigated.

Your sentences are grammatically correct but they don't fit because they conclude that something has happened. The thing is, we don't know for a fact that it did happen.

Hope this helps some.

EDIT: I just read the other posts. Aside from what has already been stated in the other post about 'expression of disbelief'. It is also a matter of intonation, emphasis and context.

Consider this:

/Why would he steal my money?/ - uncertain if he is the culprit

/Why would HE steal MY money?/ - uncertain and stressing ownership

/WHY would HE steal my money?/ - disbelief that 'he' is the culprit

Although all three are written the same way, from a written dialog it would need to be deciphered by context. In a spoken dialog, it would be deciphered by tone and emphasis (and context.)