I am sorry for your situation.
Your fiance is getting married with a British Citizen just for the sake of citizenship and this kind of marriage is called 'marriage of convenience' and I am pretty sure the British Immigration officials ( or the Entry Clearence Officers, depending on where does your fiance apply for the papers ) will be able to point it out easily so he will not be able to get a visa. Moreover, even if he is able to get through, it will still take him a long time to get a British passport for example if he gets the visa today, it will take at least 4 years to get British citizenship.
As for your other question, it is not allowed in UK to have two wives at the same time so if your fiance marries that woman, he will not be able to marry you unless he divorces her or she dies. So there is no question of your getting a dependent visa.
And if he divorces his wife before he gets a Permanent Residence for UK
( which will be after three years of marriage ) his visa will be cancelled and he will have to leave UK.
I hope I have answered all of your questions.