That's a very tough situation @OvertiredandConfused - you've come so far and done so well. I completely understand why you're so anxious. Have you been with the same provider for a long time? If you have a good relationship with them maybe it's worth talking to them about it? You could of course also keep ordering as someone has suggested - I'm never sure how much they actually bother looking at our medical records (though recently a friend of mine got told by Numan that they wouldn't prescribe to her any more - after almost 2 years! - because they'd noticed something on her records from when she was 19!). I'm a gallbladder-free gal myself and many of them will prescribe - a few knocked me back but not many - but as you say the issue for you will be if you are still too light to 'start over'. I wonder if there's a way you could have a conversation with your provider, explain the situation, and somehow get 'put on hold' until after the op? You can't be the first person this has happened to! If your doc agrees to support then that's even better.
It's very frustrating when you've worked so hard. But the pain of gallstones is intense, and you can't live with that, so the op is non-negotiable - this feels big and scary right now, but it also means an end to that terrible pain.
Funny though how they just don't 'get it' - if you found it easy to follow a low-calorie very restricted diet, you probably wouldn't be on MJ in the first place! Hopefully you'll get it done soon - is it keyhole? If so then you'll recover quickly, and can see where you are afterwards.