I just did it as the last planet before the Enclave. Can i ever go back?

You return to fight a siege, either for or against Queen Talia. After that you can never return.

Supposedly yes, but you would need some time for it. So that is too late.

No.

 

(edit - in terms of shopping/quests)

But the Queen said you could come back again later. I thought I read somewhere you could return again. If not, no big deal.

Once you leave Iziz/Onderon the first time you can't ever return 'normally'. You can't return to complete quests or play pazaak or...etc. You just return to fight.

I have finished the Civil war quest and after I cut the queen down like a dog General Vaklu said I would be welcome back once they had cleaned up after the war.

 

So I can never go back?

No, you can't.