Re: Deadlift and lower back issue...
Dare, sorry it's taken me a bit to answer this last bit.
I asked about height because if you were 6'3" or something, that might have an significant impact on form, but at 5'10" you should be able to do this pretty comfortably.
I think you are able to get into that position, but that you lack strength in that position in the hamstrings and glutes and end up taking most of that load in the low-back. When you shift your hips down and start with a higher chest, you're able to mitigate that weakness by bringing your quads into the picture a lot more.
I would definitely not stop deadlifting, but what I would do is to make sure that you're adding accessory exercises in after your 5/3/1 sets that make sense to address your weaknesses. Back extensions, hip extensions, Glute-Ham Raises (or similar), Good mornings, anything that can help to build some strength down there.
For your 70-80-90%, make sure you recalculate these based on 90% of the 1RM you calculated, dropping down a bit will lower that percentage just a bit and help you with being able to hold form better. So if your 1RM is 200 lbs (just using an easy number), you would base 70-80-90% off of 180lbs.
Stop and reset for every. single. rep.
When you set up, stand over the bar and look down at it. The bar should cut your shoelaces in half (just below the knot). Now, hinge your hips backward and reach down and grab the bar. Rock your shins forward until they are lightly touching the bar. Now lift your sternum while keeping your shoulder blades down and back (squeeze the lats). To start the lift, squeeze back into your heels, hamstrings, and glutes and lift your sternum. You should feel the primary tension in the hamstrings and glutes. You can try this with an unloaded barbell until you feel confident that you're using the right muscles to perform the movement and you'd be surprised how challenging it can be.