Could someone explain when you use bra and when fin, please?
While fin and fine are cognates, they do not mean the same thing. Fin means beautiful, while you'd use bra for fine or good.
What's incorrect about 'Allt ser bra ut'?
Se bra ut = to look fine
There is an overlap, but not synonymousness.
When to use allt vs allting??
They're essentially interchangeable. "Allting" is slightly more formal, but that's very heavy emphasis on "slightly".
Is there anything wrong with "Allt verkar fint"?
Fine and fin are not necessarily synonymous.
why i cannot say "Allting ser ut bra." here?
You can never say that because of the word order, it must be "Allting ser bra ut".
Funny, I always use this word when I speak Swedish, but I thought it was wrong. Like this was just one of those times where I turn Danish words into Swedish :-P