Is there any way to restart the Diplomatic Immunity quest?
Solution 1:
There is no way to restart this quest.
However, there are couple options available:
1) Load earlier save, then give the guy the appropriate gear (remember lockpicks!)
2) Sneak around in the mansion and get some gear.
3) Lower the game-difficulty and beat it.
You should have no unsolvable problems doing 2 and 3 together.