I know that you like the HIT ideas but your current equipment situation makes this a difficult task for legs.
I would suggest the the sets across approach for your db squats.
You said you can get 15 reps in one set. Try doing 5 sets of 10-15 with the 80lb dbs.. When you get 5 sets of 15, go for 5x15-20.
You can hit legs twice a week, once with the squats as I described. The second work out do a single leg variation like a walking lunge, rear foot elevated split squats, reverse lunges etc.. I would strongly recommend you use the sets across volume approach for the single leg work as well. Balance becomes an issue when trying for that one all out set and you inevitably end selling your self short.
Most important when using an approach like this is to pace yourself. Don't go balls out on the first set, last set should be the hardest and you should be able to get at least 80% of the reps compared to your first set.