It may be a long shot, but I hope it is just behavioral issues. My Lacey, four years ago starting drinking and peeing like crazy. Her urine specific gravity was messed up too. After a few quick tests we waited a few days to see if it was behavioral, then they did more testing.They were leaning towards the same things on your list. Found nothing in the test results, and a few days later she stopped drinking so much and then peeing less.
The reason we came up with? My RB Mandy had to be leashed (due to her bad heart)instead of running loose with Lacey. It must have upset her, there was nothing wrong with her, she was back to normal.
According to some of your recent posts, moving and all, could it be Romeo is just upset? I hope that is all it is.