Overall I do like the idea here for Microlite, and not only because I’m the aforementioned player!
Particularly if you’re playing a more “by-the-book” m20 rule set, allowing other classes to use scrolls (and I’m thinking both arcane and divine scrolls here) can give them a little extra buff without being overpowered. It would also lead to some cool tactical decisions, like “give the fighter the scrolls of protection to tank more effectively,” or “give some divine scrolls to the magic-user so he can assist in healing while lobbing artillery from the back row.”
As pertains to using HP, I agree here that one of the benefits of scrolls should be that they don’t require a sacrifice of HP as normal casting does.
As pertains to the DT, the formula looks good at a glance, it will be interesting to playtest it bearing in mind the PC might have crazy modifiers in the applicable Ability or Skill.
As pertains to the consequence of failure, I do see the point presented here. I could offer two options: 1) for low-/medium-magic campaigns/worlds, the scroll is consumed on failure, because magic in general is a wild and unpredictable thing, or 2) for high-magic campaigns/worlds, failure means the spell is cast but still does tax the caster in the form of HP loss, because even where magic is somewhat refined, unskilled casters doing it wrong still suffer consequences. And I’d wager the fighter who manages to cast a fireball to take care of a whole slew of enemies will be happy to drop the 7 HP it cost, rather than having to cut his way through them.
As pertains to magic-users/clerics using scrolls, I’d propose that using scrolls of their own type of magic automatically succeed, and they gain advantage on using the other type. This also answers the question on why a magic-user would create an arcane scroll–it will always work.
As pertains to where to put these rules, I’d suggest a reference to the ability to use scrolls in the class descriptions (M-U/C), and the majority of the mechanics wherever the scrolls themselves are listed.