I guess I'm kind of OK with that? I mean, fireball is a legendary spell in E6, and it's not like 0 HP is particularly dangerous in 5e.I agree with you on your key points. Remember we have to think about other classes for HP. At 5th a wizard or sorcerer could have 30 HP assuming primary in Int and secondary in Con. At 6th, 36. A bard or rogue may well go primary Dex/Cha, secondary Cha/Dex (or Wis), so they might well have at 5th 25HP, and 6th 30 HP. They're unlikely to boost Con given their interests. At 5th one fireball will do enough damage on average to incinerate all four.
I think the more meta-game goals for broad acceptance of a 5e E5/E6/E(X) would be as follows.
1) Make sure it's compatible as possible with existing 5e rules. (Although my personal favorite version of 3e was a total mod using E6 as a base. Gnorman's Complete E6 Compendium)
2) Have alternate advancement, not diminished advancement. I mean, you could do E10 with everyone getting Hit Die every other level and spell advancement every other level, so that all those fun Tier 2 abilities are in, but I don't think that would feel as good. If you want to keep the demi level idea, maybe make a bunch of 4-5 level "prestige classes" that don't give Hit Points, spell slots, or proficiency bonus, but have some cool tier 2 grade abilities instead.