Translation:The girls return to the restaurant and sit back at the table.
"Return to the restaurant," not "into." And "sit back to the table" might be considered acceptable but "sit back at the table" would be better. (Try googling the two phrases, in quotes, for comparison.)
"sit back to the table" would not be acceptable in the English I learned, and "sit back at the table" sounds wrong to me as well - "sit back down at the table" is what I would say or, if you are not talking about the movement towards the sitting position but about the resulting sitting position, "sit at the table again".
Not a lot of difference. Visszatér is a more general "to return", and visszajön is "to come back", i.e. spoken from the viewpoint of the place they are returning to. If you're outside of the restaurant and watch the scene, you cannot say visszajön, but can still say visszatér. Or visszamegy in this case.
The English is all wrong, we would say "... to the restaurant .... sit back at the table .." You can't sit "to" a table.
Tér is a verb which means "turn" in certain senses, including the physical sense ("turn to the right") and also less literal ones (like "the conversation turned to a different subject".)
With the prefix, visszatér means "return".