Hi, iv read most of the posts but only skim the last few, so sorry if iv missed some bits, but just to offer you a chance of hope...
My 7 year old bun (not a giant, a lop but not sure what breed, shes 3kg) had paralysis of her front paw. I know this is not exactly the same as what your buns got, BUT after almost 4 weeks on metacam then reducing off for about a week and a half, she gained feeling back.
Like your bun, there was no apparent cause, x-rays were the first thing to be done etc, the vet did the deep nerve test and no feeling. She couldnt walk on it she just held it up. After a few days on metacam, she started to drag her paw whichever way it fell, then by just after a week started walking with it the right way, and by 3 weeks was hoping around crazily. She had the deep nerve test and all is back to normal, BUT the very bottom of the paw, under the bend, isnt 100% right, her toes are spayed out and she cant clean herself, but she doesnt let that stop her. This was a few months ago now, and since has had a check up and she hasnt got worse.
My vet put her on a dose of "6 drops", 2* her weight in drops, so for yours it woul be 13 drops. Its the dog branded metacam. She said the dropped that comes with it isnt very accurate, and just to squeeze the bottle gently when perfetly vertical upside down, and a full drop will fall off. I did this into a bowl and then used a syringe to suck it up. It wasnt very much, nowehere near even half a ml, tiny amounts, but it seemed to do the trick.
My vet initially thought she had trapped her nerve, but it didnt show up on the x-ray, but after the metacam she was convinced, she said nerve damage can take ages to recover, and in some cases never recover, so with the way my bun recovered ut not fully, she thinks it could be that. Not sure if that helps you in anyway, but dont give up yet!
How long has your bun been on metacam?