To be honest? I think part of the problem is very likely that she looks quite obese, coupled with her SM and existing neurological damage pre-surgery -- her hind legs may simply now not be able to manage the weight she is carrying in any coordinated way, if she already has neurological deficits.
I know a heavy westie who nearly had to be put down because he was carrying a lot of extra weight which had caused spinal damage that was making his hind legs very wobbly in a similar way -- you could see the effect in an xray and it was very serious: the vet was not even sure he would not need to be euthenised; his one chance was to take the weight off and see if he would recover some ability to move around (he couldn't by then even raise his leg to pee). He is still around and lost about a third of his overall weight, but has permanent disability from the issues the extra weight caused.
Have you talked to an orthopedist about what may be going on? I just wonder whether this anything at all to do with her SM and may be her weight; or if there's a cause and affect with the weight maikng it hard for her to manage to walk if she already had existing damage.
Also she seems unable to get any kind of grip on the smooth wood floor which won;t be helping the situation -- I'd be sure to clip out all the hair from between her foot pads etc or have a groomer do this so that she isn;t sliding on her fur, and also then get something on the floor like those plastic walkway strips you can place on carpets, or area rugs/runners. My friend did this for her elderly labs when they found it difficult to get any grip on a wood floor and she was afraid they would fall and hurst themselves. Is Abbey finding it this hard to get to her feet when on carpet as well?
If it turns out she cannot recover the use of her legs there are small carts you can get for them but this would be very hard for her if she is hauling extra weight -- you'd want to get her quite fit and lean.
For her health overall and to help give her every chance to recover some ability to walk normally, I would really focus on getting about a third of her weight off gradually. Especially if she isn't getting much exercise given her disability, I'd maybe halve her food -- she cannot burn off the calories she would have had as a norm in the past.