I'd rather see it the other way around with the invis spell. The annoying thing about it is that when a warlock or thief uses it, and then they do speed/sight that the spell is canceld. If they do an attack they should become visible imo.
Well it was rather worthless the other way around, all it is is an escape ability which doesn't work well with AI.
If you want melee to be ignored, then make sight work for every creature that is near the creature with sight. So that they are visible for the creatures and maybe within a line of sight for the keeper too.
Well I have no idea how to do that. I made Melee not possible to reveal invisible creatures only because it is nice and basic, I asked DL on how. And from what you said only Mefisto is capable of that, but he can only go so far.
Don't worry recharge and effect time is changed too to balance it more.