You can enter such dungeon once and reset the vendor stock multiple times (usually by going back to previous level of the dungeon but sometimes there are some nuances to it) to farm good affixes on one skeleton key.
He’s more complicated. I think that the probability for him to drop nothing, since there are 3 separate drop pools, is as follows:
(1 - 50.05%) * (1 - 30%) * (1 - 8%) ~ 32%
Because these 3 drops are independent events.
You should be able to get multiple items too.
For example one item from each of the 3 groups but it would be pretty unlikely because just War Shield is already quite unlikely (8%)