Poor you and poor Amelia!
This is not something I’ve come across, but I just found this on a google search;
http://m.petmd.com/rabbit/conditions/urinary/c_rb_urinary_incontinence
The article sites a few causes which you can rule out immediately (like the things you’ve already tested for and the poor living conditions it mentions as it sounds like Amelia has a lovely home!)
It does talk about kidney stones which might be worth checking, as well as spinal injury-is there any chance she might have caused herself an injury which you didn’t see, buns are so good at hiding these things.
This article about bladder stones may also be of use;
http://www.rabbit.org/health/urolith.html
Like I said, it’s not something I’ve come across, but a kidney problem could be a route to investigate. Both articles talk about ‘sludge’ which sounds similar to what you’re describing as well as a lack of awareness that they are even urinating. There’s a chance that Amelia hasn’t become ‘un-trained’ but rather she has lost the ability to recognise she is urinating.
Best of luck to you.