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 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:
Example (J) is ungrammatical. It uses a noun phrase after the preposition because.
Hope this is helpful!