Well, you could do this with a custom DogTag for your target frame, but that would be a bit complicated. You're right; for the functionality you want, the best way to achieve it is to avoid installing LibMobHealth.
Unfortunately, it's not easy to just add an option to disable LibMobHealth - it's a library, not a module, and libraries are not designed to be loaded and unloaded dynamically that way.
I don't have LibMobHealth installed, and I'm noticing the same behavior - my targets are dynamically updating a SureHP value as I attack them.
As above, I'd like to have [Status | SureHP | PercentHP:Percent] work and not return static health values for mobs/bosses.