I am so sorry ... I cannot even imagine how frightening and overwhelming all of this is for you.
The only advice I can give you is to take it a day at a time. You really don't know what tomorrow will bring..no one does for sure.
As for will she be happy if she is not able to regain use of her hind legs....I believe she will be. I have seen many pups end up in wheelchairs and be happy go lucky pupsters! But...you are not there yet. Until they tell you that there is no hope, I would keep praying and hoping. She has been through a lot and is under the influence of a lot of medications. My Maxwell had a 50% chance of breathing without a trach after his ordeal and he did pull through....so there is always hope. Hold on to that for now and try not to think past where you have to go.
Lots of prayers are being said for Layla and for you!