For the second question, (tbh I do not now very much about DGs hehe) but how about just showing a message like you're already doing but instead of casting an actual spell just setting her HP using trigger vars?
Something like:
Code:
eval perc (%actor.hitp% * 100) / %actor.maxhitp%
if %perc% < 90% && !%actor.fighting%
%actor.hitp% = %actor.maxhitp%
endif
Haven't tested that code and again I'm not that good with DGs, just an idea
For the first question, do you already have a spell that does that? Asking cause dg_cast() needs an existing spell so, does the 'sovereign heal' spell already exists or are you asking for directions on how to create that spell?