Can I book a Eurostar round trip ticket if my date of return to London isn't a day return but instead a couple of weeks out?
Yes.
absolutely. That's what most people do.
Regular tickets these days don't have a stay requirement; you are effectively booking 2 singles.
Unless I am reading their site wrong, there does not appear to be any price savings for a round-trip, other than if using a credit card the surcharge is for the entire transaction.
Eurostar abandoned roundtrip discounts for about a year, but now they're back again. The web shopping cart should offer the best rates for each leg. (BTW, into one city and home from another still doesn't count as roundtrip.) http://www.eurostar.com/uk-en/travel-info/travel-planning/eurostar-fares-and-fees
Yes there are some relatively small discounts for booking each way at once - not like the old days when the single was ridiculously expensive. Although in reality it was done as an increase on single fares back in April.