It is wonderful the metacam is helping her so much. Is it possible she has arthritis?
It could be but it seems strange it came on so suddenly. Although sometimes we only notice when we see them fail to climb into something, but it does sound like Lilly's had a fairly sudden onset.
I hope your vet and her instinct works out what it is. Good that the Metacam seems to be helping. As you know Pip has also developed weak back legs, but not as severely - it's only with climbing, and he is older, and I don't see that as unusual with an older hamster. Our first Syrian, Charlie developed this - I can't remember how old he was - probably nearly two. He lived to two years 8 months and would zoom around on the flat, dragging his back legs behind him - still full of energy and life and would hop on his wheel and run for about 10 seconds once a day - just to keep his hand in perhaps
The vet at the time thought it might be Cushings but I wasn't convinced. In hindsight I think it was an internal tumour pressing on something as near the end he was developing a prolapse of the testicle area.
There could be a number of reasons and a mini stroke could be one of them, but with a stroke you'd normal expect more than just the back legs to be affected and the recovery to be different and perhaps quicker.
But it's good she's enjoying her scrambled egg and usual activities. It's good the Metacam is helping a bit. The fact she seemed surprised her legs wouldn't do something sounds familiar and suggests she is not suffering, but if the Metacam helped at least you know she is having some pain relief.