I think you underestimate the effect of 'than' in the sentence, but with 'than' the sentence is no longer a classical conditional sentence like:
migrants would still be better off if they stayed at home.
With 'than' it's a comparison of two situations (scheme: A is ..er than B): a real one (even if yet to come) and a no longer possible one ('if they had ...
I don't know how late I am, but I want to reply this:
One way to explain this is 'been' is the past participle of be and should be followed by modal auxiliaries.
I think it is easier to explain this by using rule says that 'been' can only be used after 'have' in any form of this verb. That means first sentence is truly wrong.
A conditional sentence can use the future tense in the second clause:
If you study hard, you will pass the exam.
If I have time, I will visit him.
This is called by teachers, a conditional sentence, type 1 used for future, real situations or facts.
Teachers usually teach present unreal or future conditionals as given in the OP's examples:
If you didn'...