I use the term "fiance visa" for customers on JA because it tends to be easy to understand and quicker to provide an answer. In reality the application you make if you are a fiance OR if you are a spouse abroad is the same - you are applying for what is called "settlement". The same form is used for both application.
The difference is the visa that is granted; if you are fiance you will get a 6 months visa so that you can come to marry in that time. A spouse would receive a 27 months visa.
I agree that it is silly to ask for booking of a venue etc but it is about the interpretation made by the UKBA for the Immigration Rule that states you must intend to marry within 6 months of making the application if you apply for settlement as a fiance of a UK national. There is a big problem with overstayers in the UK and the strict interpretation is borne of that issue. There is not a lot of getting around it and it is certainly worth playing their game and providing evidence. It will get him here quicker.
Gilbralter is generally regarded as the easiest place for foreigners to marry. There are no residential requirements like there are in most other countries. This means that you can marry there provided you have a special licence (which is not actually that special or difficult to attain.
You can arrange this with the general registry office of gibralter before travelling. You can contact them for further details here:-
00 350 20042706 (f)
The best thing you can do is to instruct a solicitor to prepare which ever application he makes. Your MP will write a letter, nothing more. This will not get a great deal of traction and, though I am sympathetic with your daughter's position, in the final analysis a solicitor will assit to get him here quickest.