I wouldn’t say you are wrong about the rounded back, but I would say that someone who hasn’t lifted atlas stones shouldn’t start with that. Learning the “correct” deadlift form to learn bracing and to strengthen their back/legs from years of not doing strict back workouts would be advantageous before going to town on a 50 kg stone. But yes, get to lifting now and feel less pain later!

Hip hinge angle is the angle at which you will have to bend over to grab the barbell. Depending on your skeleton morphology you could have a near horizontal back when grabbing the bar. On the other extreme you will sometimes see lifters with an almost straight upright back lifting. Most people are so where in between that angle though.

Your hip flexor is a muscle group connecting your torso to your upper leg skeleton. These help to either lift/flex your legs or torso depending on which is stationary. Deadlifts can work the hip flexors, but a better exercise would be something like leg raises, lunges, or even squats as those exercises would target more of the front leg muscles. Good luck!

Different people have different hip hinge angles depending on their genetics/morphology. So some people can have a lot more upright back angle while some will almost look horizontal at start. So long as the spinal erector muscles are rigid, and there is minimal flexion of the spine during the lift, then the lifting force itself will be predominantly coming from the hamstrings not the spinal muscles. The back muscles will be worked, but working to remain rigid and not to move the weight itself.