OP, with respect, it sounds like your surrogacy agreement (nb not a contact) is slightly superficial.
I can't believe you don't have a firm answer to the issue of a twin pregnancy, surely you have agreed whether they can opt for selective reduction or not? This is basic, basic stuff.
I'm also surprised at your answer about the child 'legally' going to the biological father if they split up. That's simply not true, there is no presumption in favour of a genetic parent in English law. The second father (psychological father whatever you want to call him) is of equal status to the first once the parental order is made. And until the order is made you have more authority than both of them.
Also I think you're being over optimistic about how 'above board' your expenses are. Given that you've mentioned a 'recommended amount', and that your 'expenses' are being worked out in advance what the intended parents are likely doing (like hundreds of others, egged on by lawyers and lobby groups who want to legalise commercial surrogacy) is presenting the court with a fait accompli in the region of £15k and putting the judge in the invidious position of having to retrospectively authorise it.
You've probably been told it's fine, everyone does it, and yes.. if you're lucky you might get a parental order reporter and a judge who is willing to nod it through - but if you're unlucky you can expect a LOT of scrutiny and a potential rollicking.