Some of them, most of them

How are you, everyone?

1) He has a lot of friends. Some of them are from Europe. (o)
2) He has a lot of friends, some of whom are from Europe. (o)
3) He has a lot of friends, some of them being from Europe. (x)
4) He has a lot of friends, some of them from Europe. (x)
5) His friends, some of them being from Europe, are quite interested in Korean history. (o)

I learned that this "absolute construction" option is available only with medial, not final, relative clauses (and only if the grammatical subject of the medial clause is the same as that of the main clause. Whereas a medial relative clause provides background information, a sentence-final relative clause provides major information--information that should not be reduced.

While above 3) and 4) seem to be incorrect by above rule, however, I see frequently those kind of absolute construction with "some of them, most of them, all of them, both of them, few of them".

1. Do above examples 1) to 5) with "some of, most of pronoun" belong to 'the non restrictive modifier' or 'absolute construction'?

2. Are above 3) and 4) really incorrect grammartically?

3. If both are correct grammartically;

1) what's the difference between 3) and 4) with/without being?

2) would you introduce me the general rule with conditions to make 'the non restrictive modifier' or 'absolute construction' especially with "some of, most of pronoun"?

Eagerly waiting for your clear explanation,

Best RGDS