Infinite Invisibility for the DM is easily doable (by just setting their Invisible duration to -1). Summoning monsters should be somewhat straightforward as well, if you just give the DM a ton of "summon custom monster" spells (that may or may not be able to summon additional monsters at the same time, like how Boreas' summons work). Such summons could have nearly infinite range as well (range could be bigger than the map-size), although Line of Sight might be an issue. I don't think there's a good way to set their AI (unless you delve into the summons' charScripts), especially not mid-game.
Most likely the DM would be a "member" of the adventuring party. I'm not sure what happens if a Player-controlled character is removed from the party via a Story script (which should prob. be a good experiment). There could be different items added to the DM's inventory at the start that teleport the party to different triggers when used.
I didn't mean exactly what i said. Yeah invisibility is easy but. You will still be centered on the terrain. It wouldn't be like you're just a camera. And the spell circle will still be centered around you.
And when they removed it causes a black screen on whoevers controlling it i believe. Or just causes them to lose visibility of everything. The main point is that you dont retain control of the character.
A better experiment is setting the alignment to evil npc and seeing if two player control characters can fight.
