No chance to change it on-thefly. Osiris needs to know a character at compile time. Spawned characters don't exist at compile time.
I think adding a new global character to the game, even if it did not make the game crash, will not work. My own experiments with changing objects from global to not global resulted in crashes, I think because the global chars are stored somewhere in the save files and the game crashed when it did not find them anymore. So I also think that adding a global character will not add it to an existing savegame. (But that's speculation as always, no real knowledge ;-)
You can experiment with creating a summon skill with infinitve summon time, try using -1 for the number of turns variable. A summon should count as a party member and follow the summoner wherever he/she goes. If there should be a way to dismiss this follower, you'd have to kill it by script and you'd probably need a second skill or item for this. And the summon would probably require a special charscript to identify itself against some event.
I don't remember though if summons really auto-follow through portals.
The disadvantage of a spell is that it's executed by the engine and you have no control over it.
If you'd need no despawn, you could use a scroll to cast the summon spell from so that you avoid problems with multiple skill uses.
The other possibility would be to pre-place the follower and never spawn it but teleport it around when needed (if it should be 'despawnable'). Such a character would never auto-follow through portals though. On 'dismiss', you'd simply CharacterSetOnStage( 0 ) it. I don't know if setting the char offstage would deactivate the follow script though.
You'd have to modify the resurrect skills too to be usable on non-party members, or introduce a follower-only resurrect skill. You can look at the Death Knight Bane spell as a sample for a spell that uses 'tags' to restrict use of a spell on a specific target. (in 'data "TargetConditions" "..."' I think; must be a spell of type 'target' then of course, but I think all resurrect spells are of that type already.)
If you don't care about 'abusability' of the regular resurrect spells, you can modify them to work for non-party npcs.
For your object script questions, some knowledgeable modders will show up. I only ever used object scripts when there was really no other way to do it or when doing something in Osiris would have been far more complex than an object script ;-)
