Dominion: De Gilden

Coin tokens come from the token supply (not other players); before buying in your Buy Phase you may spend any number of them (each gives +1 Coin) and return spent tokens to the supply; substitutes are allowed if you run out (page 3).

No — Coin tokens can only be spent before buying cards in your Buy Phase, so tokens you receive when you buy do not affect that same buy (pages 3, 7).

You may pay additional coins only when buying a card that has a “+” cost; you must actually pay more than the cost to overpay, and those coins are spent (not available afterward) — the “+” is just a reminder and the card’s normal cost still matters for other effects (page 4).

No — overpaying is only possible when you buy a card; you cannot overpay when gaining a card by other means (page 4).

Yes — when overpaying Doctor you shuffle your discard pile into your deck to continue looking, and that reshuffle includes any cards you already discarded to Doctor’s overpay ability this turn; if there are still no cards, you stop (page 6).

If two things happen to the same player at the same time that player chooses the order to resolve them; simultaneous effects to different players are resolved in turn order starting with the active player (page 4).