I have successfully shifted a blockage this way before and am only suggesting you try it, as your vets seem to be going down a really drastic route, but as people keep reminding me, just because something has worked for me, doesn't make me an expert, but your vets option is as final as it can be. The water needs to very just warmish, because my vet said cold water from the tap is to much of a shock to the system and kind of defeats the option. Room temperature is also fine. Keeping bunny warm helps keep them nice and relaxed as well which will help with shifting this. If you look on 'You Tube' 'How to massage a rabbits tummy', I found an excellent one which I followed with my bun. Basically you mustn't be too hard, but you need a little bit of pressure for it to be effective and instead of going round and round in circles, it was more like stroking from beneath ribcage in the direction you want the blockage to start to move, but I did some circles too. What really helped as well was how the lady was very gently and slowly stretching bunnies back legs out as well. She would bring a leg very carefully up to tummy and then very slowly extend the leg out, and a couple of times I felt a physical shift. Initially only tiny bits of poo came out and it can feel like you could be there all day with nothing much happening, but even tiny bits are better than nothing, and EVENTUALLY enough tiny bits will pave the way for larger amounts to follow. Again, this is only what happened for me, but it might just be worth a try.
after i did that with some warm water i got 2 poo