Re: lower back issue in heavy deadlifts
Your lower back is weak, abs are probably weak and you don't know how to stay tight. During the entire lift you were worming around, it wasn't just your lower back rounding, you just weren't tight.
You need to learn to get a flat back position, tighten up your abs and fill your stomach with air pushing out against your abs, back, entire core area until you feel like your face is going to explode. That's getting tight. A belt will help you feel this as your abs can push out against it.
Back extensions, 45 degree back extensions, good mornings, RDL's, deficit DL's, band pull throughs will all help strengthen your back. Also add in a lot more ab work.
What does your strength programming look like? If you guys do a standard LP where you DL once a week, then I'd suggest picking one of the exercises above instead of DL's and working up to a heavy 5 reps, then add 5-10lbs a week for 3 weeks only going as heavy as you can keeping that flat back position. After 3 weeks, switch to another one. Once you fix your lower back weakness issue, your DL will skyrocket. No, you don't have to do DL's to increase your DL. The band pull throughs can be done a couple times a week as a finisher. Just do 2-3 sets of 20-30, slowly adding sets and reps and increasing band tension.