I don't even think about squats with my back issues. Maybe one of these days as I get stronger.
I'm doing some of the above exercises religiously with great results. Mainly the leg press, leg curl and extensions. My thighs have a lot of definition and more mass in only 10 weeks.
I do 3 sets of 12, 10, 8 reps as heavy as I can complete the last of the sets.