3) The Defender's HP compared to damage....varies a bit:
For example, a 5th level fireball is doing roughly 5.6 damage per level. A d6 character with a decent con, and d8 and d10 characters are going to generate scale faster than that. At the top of the chain with meteor swarm, its 8.2 damage per level...so generally your d10 characters will keep par, d8 and d6 characters will fall behind the curve a bit.
But hey this is all math and stuff...lets look at an example that might put this in perspective. A 20th level fighter....against an 10th level wizard. Cuthgar the brave 20th level fighter, takes a walk out of the inn due to the "call of nature". Bosniak, the evil 10th level wizard, sees Cuthgar and begins his fiendish plan!!
Bosniak casts Dominate Person on Cuthgar. Now the spell actually gives some very easy ways to force disadvantage on the saving throw, but lets just look at the straight up numbers.
Bosniak: 19 int (starts with a 15, +4 for ability increases is a 19. He's an NPC so lets not top him off at a 20). Prof Bonus: +4.
So his DC is going to 8+4+4 = 16.
Cuthgar: 16 wis (I personally think a 16 is about the top end wisdom you would expect for most fighters, unless the fighter is going for the true "wise warrior" motiff. I certainly don't think anything higher is typical, frankly a 14 to 15 is more likely). Prof Bonus: 0
Wisdom Save: +3
So Cuthgar has a 40% chance of success against a wizard half his level...and Cuthgar is no slouch in the wisdom department. However, Cuthgar has the indomitable ability. If he uses it (because no one is going to take kindly to be mind jimmied when they are on the john!) his success rate goes up to 64%.
So...considering there is a 36% chance the Fighter falls completely under the control of someone half his level....would people say that is an issue or not?