Under EASA commercial rules, we can go the full range of the 650ER and more with 3 pilots (18 hours duty). However, the rules for the crew rest are a bit more specific: to be an approved crew "bunk", there needs to be a full-flat bed, light proof, with adjustable temperature, and reasonable sonic insulation. As the rest is right next to the forward galley and the forward lav, I need earplugs however to catch some Z's....
The big "cheat" is that there is no space for a second flight attendant, nor for the first one to have a decent sit-down when the crew bunk is occupied. So most operators don't have FAs but "ISP"s instead (inflight service providers).. They don't have official emergency duties, so they don't need proper rest or training..
If I owned a 70M$ airplane, it would bother me that the person preparing my food and looking after my well-being, as well as being closest to me when things go pear-shaped, is not subject to flight duty limitations...