Send BTC with Electrum: which address is used?
Great question, I've always wondered this too. But im pretty sure that Electrum, by default, will send your oldest unspent outputs first. (Sorry if example is a little repetitive)

E.g. If you recieved 0.4 on address A 3 days ago, 0.2 on address B 2 days ago and 0.1 in address A again 1 day ago, it would send 0.4 from address A, then 0.2 from address B, then 0.1 from address A again.

Im pretty sure you can change this in Dev mode. I'll look into it.


As I said, in order of oldest unspent outputs (balances)
Yes, they can send from multiple addresses. But it will try to send from one address if possible.
Try the coin selection setting in settings.


EDIT - Ah, yes you can get it to spend from certain addresses. Go into your Electrum settings, and set your "Coin Selection" mode to privacy.