I think the family thing is marriage and ya still have to pay and get lots of proof and documents. If this is the first time your girl as been to the UK ya may be better going to an agency.
I don't think the idp is mandatory I done mine without albeit a few years ago.but I think now if on tourist visa they do give the 5 year license when you renew your original 2 year license.
you could be right, its a long time ago for but I do recall I had to prove my income Apr to sponsor,then it was only £18,600 now it's 27k+ and soon going up again.