Question: Is slab-reefing (jiffy reefing) and lazy-jacks genuinely viable for a 667sqft sail, given we will be only two sailing our boat most of the time?
Answer: Sure, perfectly viable: We have sailed over 80,000 miles with such a system on a mainsail that is only a little smaller than the one on the boat you are considering, but the system must be well thought out using top quality gear and powerful two speed winches. Even then, it does take a fair amount of grunt to reef. I am 6′ 2″, about 190lb and fairly fit; a lighter/smaller person could have problems.
Our main is 620sqft and has full battens. We fitted a Frederiksen luff track system some years ago that makes reefing much easier, particularly off the wind.