I actually quite like some of these. They certainly go a long way towards lowering the power level of players, easing up on the cheese. I also definitely love the "half proficiency in all saves after 6" feature. Means the Barbarian might actually make his intelligence save against that Ulitharid stun.
Personally, I disagree with a few of the nerfs you've applied to your players but I understand why you've put them into place and I wouldn't argue against them if I found myself at your table.
Overall, good stuff. One thing I would add to your multiclassing rules is an addendum: "Remember: you're playing a person, not a stat sheet. You don't sell your soul so that you can add your Charisma modifier to Eldritch Blast."