OP, this bridezilla really is taking the piss here.
I get the impression the groom is just going with the flow for an easy life so he is unlikely to be unreasonable if your DH tells him you can't attend due to finances and he really should tell him.
The coup,e have moved the goalposts by changing from a UK wedding to a ceremony abroad, so you are entirely justified in saying you just can't do it now.
I do think they should pay for you to attend. I know others disagree but I believe that if you have a wedding somewhere that the majority of people can get to easily and where there is a choice of accommodation in various price brackets, then it's fair enough to expect people to pay for themselves.
As soon as you go down the route if hiring a venue for exclusive use, ie there is a fee to pay and your fee will be reduced if others contribute, then you are totally abusing your friends good nature. In that situation I think you foot the bill or choose another destination.
You would be insane to use all your spare cash on this wedding and it will likely taint the friendship anyway. As you said, it's not how you want to spend that money and that will be pretty hard to ignore.
Not paying anything in the foreseeable future is a good plan whatever you decide to do as it does seem to me to be quite probable that this wedding might not even happen. Losing the money completely if their plans changed would be the ultimate insult.