I have 11 years of playtest experience in a number of campaigns as evidence.They have entirely too many hit points? You have maths to back this up, I presume, since looking at the damage outputs of even poorly optimised characters and monster HP of their CR suggests otherwise. I presume you also have maths to show an increase in monster hitpoints at a given challenge rating over the course of the edition. Anyway, if you lower monster HP, that doesn't encourage 'cool new options', since it makes optimising for damage even more important. Monsters can't fight back if they're dead, and when you kill everything in one hit, you don't need anything but damage and Initiative.
Caster level is unrelated to the spell progression.By halving caster level you don't actually give these options out. Full casters don't get 6th level spells before Epic, and half-casters don't get 3rd level spells before Epic.
Wizards still get 3rd level spells at level 5, but their CL is 2 at that time.