Does anyone know if the situation in Europe is similar to
that in the US? In other words: How risky (financially
speaking) would it be for me to buy a round-trip ticket and
discard the return portion?

I did that with Ryanair, London to Rimini, no problem. Returned the date
planned, but from Geneva on Easyjet, throwing away the Ryanair return
coupon. I've also done that with BMI, returning via Easyjet from Athens
instead of from Prague (don't think BMI goes to Prague anymore).

Years ago, Swissair used to police the use of tickets in Switzerland issued
abroad to make sure that travelers didn't arbitrage fares and that they
traveled in accordance with the fare terms. Well, Swissair's arrogance cost
them their life.

I know for a fact that the ferry companies and Eurotunnel charge credit
cards the higher one-way fare if passengers and car drivers fail to use the
return ticket. Check-in is normally by means of the credit card you used to
buy online. I have no idea what bankrupts and other people without credit
are supposed to do, or whether a debit card works.

I usually pay online with Amex Private Payments (a one-off number you get
online; Visa and Mcard have them too). But the system isn't foolproof, and a
retailer in fact double-charged me once (Amex credited me instantly) which
he could do because the # is valid 30 days.