(2) The average coin cost (so ignoring Potion) of Kingdom supply cards in Seaside is $3.73 (again, if I've done my math right). What is the only set that has a lower average than this? (Again, don't care about Promos.)
"Kingdom supply" is kind of ambiguous. Kingdom cards are the 10 that are randomized. I don't think there are any Kingdom cards that aren't also Supply cards, so I don't know why that's specified. Supply card is a broader category that includes basic cards, but excludes things that you normally can't buy (e.g. Spoils).
Cards that are one of the 10 random cards you put out. So excluding things like Province, Duchy, Gold, Potion, Prizes, Spoils, etc...
And just to clarify, I didn't count Ruins towards anything, and I think I considered Knights as costing $4.9. Not that that really matters.. I don't think you will need that fine-grained of detail to get the answer right.
Oh and I guess it also probably doesn't matter, but for the number of types per card, I did the union of all the types of the card plus its other non-supply cards. So for example, I counted Tournament as 4 (Action, Attack, Treasure, Prize), and Knights as 4 (Action, Attack, Victory, Knight), etc.