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If a Daemon Prince has 5+ inv for being a Daemon. If he buys

MoT 1+ inv and Aura of Dark Glory 2+ inv can these 2 stack onto daemon inv

giving him 2+ Invulnerable save?

IIRC, MoT specifically says it cannot modify an invulnerable save to better than 3+. Additionally, Aura of Dark Glory, gives an invulnerable save, it doesn't modify one.

As above.

In addition Daemon Princes can't even buy a mark.

There are a whole bunch of ways to stack invulerable saves of a DP, but none of the ones you listed work!

Grimoire, cursed earth, sanctuary...warp storm ... legacy of the 1st war of Armageddon. .. those all affect the invulnerable save.

The mark of tzeentch works nornally but on daemons of X its just a reroll of a one.

As said the aura of dark glory is a fixed save, so pointless for a daemon prince. You can get a better fixed invun save using forewarning or a skyshield, if you're so inclined.