On the second option, if I only fly the first leg and then cancel the return leg, will I get a travel credit that I can use for future domestic travel?
Not without re-pricing to a one-way first, which would result in a credit of $0.
Any idea how much and if better to book the return as Economy (regular) or Economy (refundable)?
$0 either way — but with the refundable ticket, you could get a refund of $0 instead of a credit of $0.
You’d be able to use the return flight only on something that can be combined with your outbound — so basically a flight back to Europe, more or less.