Designing a Summer Program

Hey, I just registered and this is first post. I have been taking a weightlifting class this semester and want to continue with my own program this summer. I have done a lot of research but a few questions remain.

Rep range cycling: I want to spend as much time as possible in low rep ranges to boost hormonal activity and CNS stimulation, but I have mobile joints (can pop my hips and shoulders in and out to a degree) and apparently vulnerable connective tissue that has led to several recurring (though minor) injuries when I go to 5 reps and below for longer than 1 or 2 workouts.

Is it a good method to do low reps (4-6) for 1 workout and then high reps (7-12) for 2 workouts? I want to progress and grow fast but I also don’t want to bang up my joints.

Form in deep squats and deadlifts: I have yet to see anyone in the gym do either of these exercises without rounding the lower lumbar spine in bottom position, doing a “sacral tuck.” Not exactly the classic “round back,” more like they’re humping something. I am unable to eliminate it completely from my form, although it hasn’t resulted in any injuries. Is this accepted, what’s the deal? I am fairly mobile, although I’ve been working on stretching the hamstrings to help with this problem.

Finally, out of curiosity, is it unrealistic to try to put 50-70 pounds on a deadlift max in four months? I am assuming that my beginner gains are finished or nearly used up, but I really want a double bodyweight deadlift by the end of summer.