The upper back will round a little bit - it's just how our bodies are created. We physically couldn't be totally flat because our chests/thighs get in the way. Instead of focusing on the upper back, visualize the belly melting onto the upper thighs are you come down. Bend your knees if you have to. The important thing is the belly melting on the thighs. When we do this, we bring the stretch into the hamstrings...and when we focus on getting the hands down and we forget about the belly/thigh connection, we often bring the stretch into the upper back which doesn't do anything for our hips/hamstrings
Standing split is a great choice! Tips: lots of hamstring and hip flexor work. Practice things like lizard pose, dancer's, high lunge. Breath is boss so just hang out in the poses for as many breaths as is ok for you and visualize openness.
Be careful with this - can be tough on the knees. It may not be weakness but rather a lack of flexibility that's preventing you from lifting up. Just stay mindful in everything you do - don't force anything and don't push yourself into a realm of pain and you should be okay. But note the alignment.
I got an awful case of sinusitis for the last two weeks of March project - I only recently started breathing through my nose again and let me tell you people, it's the most glorious feeling ever. Since I was working on a dancer's pose then, I want to continue with it in April
I also want to work on my wheel pose - I have trouble getting up into the wheel when my feet are close to my hips, so I start with my feet a bit further and more open - is that bad? Can i injure myself this way? When I'm up i have no issues walking my feet back where they belong and staying in the pose for 7-10 breaths and I come down to a proper position too. Am I still too weak to do this lifting business?