If you can't do any leg exercises with heavy spinal loading I would keep doing a lot of the single leg stuff you are doing and do the best you can with that. Is it ideal? Obviously not, but it's what you got. I had a knee surgery in March and have really gotten into single leg work and I think if you go hard on it you will not lose size and can even gain size. In my experience, Bulgarian split squats are the best single leg exercise I have used for keeping or building size. There are many different ways to load the weight, such as a barbell on your back, dumbells, or a new one that I tried holding the bar zercher style. You can also elevate your front foot to make it harder.
Another thing I HIGHLY recommend with your issue is hip belt squats. This seems like a strange exercise, but after trying it I will say it really does hit the quads well, and is great because there is no spinal loading. I have used this for about six months. At first it was due to an injury, but now I just like it.
Here is a link for the belt
You can use an E-Bar like this:
YouTube - Hip Belt Squat
Or you can get a loading pin and do them like this:
YouTube - Hip Belt Squat
If you are squatting 480, you may do better with the loading pin, but either way is cool.
I realize neither of these exercises are ideal, and things like squats would be more effective, but sometimes you have to do what you have to do while you're injured. I personally believe that you can make progresses with most exercises provided you work hard on them and focus on getting stronger. Good luck man, back issues suck. I had a back surgery myself and having disk deterioration. It's pretty common acutally, and eventually I think you will find you can resume normal stuff, but as long as you're in pain I would listen to your doc and avoid anything that hurts.