I need to edit each level in a level pack separately in order to get unlimited skills from all of them, which is very hard to me.
That's a natural task for an automated script that finds levels, then edits all the skill lines. I suppose you're on Windows, I don't know enough Windows scripting to do it for you, but maybe somebody else can jump in?
Still, interesting singleplayer feature idea in general.
Player should be allowed to assign unlimited skills, e.g., to test ideas in later parts of levels. Allow the numbers in the skill panel to drop into negatives. An attempt counts as solved only if no excess skills were assigned.
Conveying why the level is unsolvable during play, that will be hard for the user interface. Probably needs glaring text.
-- Simon