@Codger made some good points. It's very hard to specificly suggest anything for chronic LBP. It's such a tricky thing. Even with knowing that it's a L4/L5 hernia on the left side with nerve root compression (whether it's posteolateral or not), there's still no specific exercise that works for all. The symptoms both neurologically and in regards to pain are also incredibly individual. Some patients show several big hernias on an MR, but have no symptoms at all. Others have a small protrusion and are riddled with pain.
This study really displays how much we don't know about especially pain yet:
I'm glad the McKenzie exercies are working out for you somewhat. I'm guessing you are on an extension protocol (on your stomach arching your back pushing the elbows/hands into the floor etc.), or do you have another direction preference? It can be a pretty good tool but still it doesn't work for everyone.
So your shoulder has luxated (popped out of the shoulder socket) several times, but the doc still says no tear on the MRI? Shoulder luxations can be a tricky thing as well. Are you only now starting to do stabilizing work on the joint?
Again, in regards to suggesting specific exercises and movements for your LBP, it's impossible to do without assessing you in person. No one can do that over the internet. It's good to hear that you have at least had some success with physios and doctors (I agree that most chiros are quacks). Just keep that up and listen to their advice! Hopefully you will get something that works well for you!
As it pertains to your back, feel the exercises out. Any increase in back pain and especially neurological symptoms (down the leg) and you shouldn't do them.
I personally don't care for the squat variation with a bosu ball shown on the video as it puts a lot of pressure on the knees and you don't engage your backside at all, but if it feels alright and it works for you, it's a valid choice. Rule of thumb is getting that backside/ass strong. If you can't do regular squats and deadlifts, there are a million other exercises out there for the glutes. Confer with your physio! Finding exercises that strengthen your lower body without aggravating your pain is the way to go. I haven't read the books but from what I've heard I don't think they're well suited for your needs.
Last thing, you say that you have very poor flexibility and can't lift your leg past 90 degrees, is that with the leg straight? If you force it any higher, or bend to touch the ground with your legs straight, do you get any pain down the leg on the backside, or anywhere else? That's for you too
@Codger