It's obvious that you've been told that arching your back is important. Unfortunately, you're really overdoing it, especially on the squat. I did a google image search for "raw squat", and these were the first 3 results that showed the bottom position-
Like I said, these were the first 3 results, so you'll know that I didn't go out of my way to find pics that support what I'm saying. All 3 of these guys are big lifters, and not a single one is arching their back. Instead they are all well braced. They do this by tightening both the lower back and the abs prior to and throughout the lift. They are all, however, leading with their heads, so don't think that I'm suggesting you do the opposite and arching and bend over. Keep a tight core all the way around and a neutral spine and I think you'll have good results.
Also, start working on bringing your elbows down to your sides in your squat setup. It's going to take some work to get to the point where it feels natural or even stable, but it's a good change to make. feel like your holding the bar on the upper part of your lats instead of your shoulders or arms. Use your lats to pull your arms into your sides. Not only will this bring your elbows down, it will also help to tighten your upper back, which is the whole point.