Look at the following conditional:

 - **James is the best student**, if only because he is never late for class.

The phrase *if only* is a way of repeating *James is the best student* with the word *only*. The word *if* in the phrase *if only* means *even if*. The whole sentence means:

 - James is the best student - even if **James is only the best student** because he is never late for class.

**The Original Poster's examples**

> G.This meeting was necessary if only for an exchange of views.

This means:

 - This meeting was necessary, even if this meeting was only necessary for an exchange of views

*For* here indicates the purpose of the meeting. In other words it tells us about a *goal*. This means that the meeting was necessary and one reason it was necessary was to achieve an exchange of views. The *if only* phrase implies that there may be other purposes of the meeting, but this was an important one.

> H.This meeting was necessary if only because of an exchange of views.

We use *because* to show why something caused something. The cause is described by the words following *because*. (H) means:

 - This meeting was necessary, even if this meeting was only necessary because of an exchange of views.

Causes usually exist or happen *before* their results. This sentence means that an exchange of views that already happened, cause the meeting to be necessary. Perhaps when everybody exchanged their views they realised that they needed a meeting. The contribution of *if only* is that it makes us think perhaps there are other reasons that the meeting was necessary but the exchange of views is an important thing that made it necessary.

> I.This meeting was necessary because of an exchange of views.

This sentence just means that the exchange of views caused the meeting to be necessary. There is no *if only* here. The sentence does not make us think that there might be other reasons why the meeting was necessary.

> J.This meeting was necessary if only because an exchange of views.

The preposition *because* can take two types of complement. It can take a preposition phrase or a clause:

 - because [of the rain]
 - because [it was raining]

It cannot take a noun phrase:

 - *because [the rain]. (X)

Example (J) is ungrammatical. It uses a noun phrase after the preposition *because*.

*Hope this is helpful!*