An enlarged kidney does not mean there is a UTI and the kidneys look a similar size and shape on radiograph so I wouldn’t be expecting it to be enlarged. What did the vet say he/she felt that suggested a UTI?
It depends what the obstruction is, it is usually compacted food, which would need to be treated surgically I would imagine. If your Vets aren’t capable of treating rabbits they won’t be able to do this surgery.
Your vets seem so unsure I really don’t know what to suggest, because at the moment your poor bunny is suffering.