If... 2nd conditional

[ozma-]

Which of them is the right one (if there is any right one)?

If someone sang, I would then be able to play a bit of accompaniment.
If there was someone singing, I would then be able to play a bit of accompaniment.
If there was someone who sang, I would then be able to play a bit of accompaniment.
If there was someone who could sing, I would then be able to play a bit of accompaniment.

[5jj]

If someone sang, I would then be able to play a bit of accompaniment.
If there was (or were) someone singing, I would then be able to play a bit of accompaniment.
If there was (or were) someone who sang, I would then be able to play a bit of accompaniment.
If there was (or were) someone who could sing, I would then be able to play a bit of accompaniment.

They are all possible, with my minor amendment to the second, though they have slightly different meanings. In the first, third and fourth, I feel that placing 'then' after the comma is more natural. It would also be acceptable there in the second.