Re: 'Hero System' online
Nah, shapechange is actually fairly easy as long as you're willing to ignore some graphics problems that occur even for player character models; you're limited to models with full animation, but that's not just PC models. It's tricky to make it do anything worthwhile (tweak aggro code?), but not hard to make it have a cosmetic effect.
Generally true.
You can still let it see through objects where an other side actually exists. Same applies to tunneling, desolid, teleport, though you'd be adding a lot of special-casing to your movement code for a pretty marginal benefit.
So? It usually winds up as 'click to bypass plot point X', but that's not totally worthless. Again, though, requires a lot of special-casing, here in quest design.
Hardly. You just use the standard lazy method of mind control -- you adjust the mob's internal faction definition and it uses its normal AI.