A furling headsail with a built up luff using foam or line will reef quite nicely even past 20%. If I have to reef beyond 20%, not too concerned with shape as seas will make beating to
weather a major problem and sail shape will
work quite well on a close reach. Have a gail sail if things get really nasty and an Asym. for drifting. The biggest problem with a furling sail is it has to be made from heavy cloth to stand up in strong winds when furled. Doesn't make it a good light air sail though way way better than a 100% working jib and a big heavy mass to handle when you drop it. The 135% that Mack
Sails built for me has done yeoman
service. Bent it on 4 years ago and its been there ever since through 40k winds in
SF Bay furled to less than 100% down to light air sailing here in Kona as well as a TransPac. The sail is like owning an automatic trans in a car, just pull on the furling line to get the right sail area for everything but drifting conditions.
To answer your first question, going with a working jib as your only headsail will work only if winds are constantly above 15k and/or you really don't care to sail and will be content to
motor a bunch. Tried to deliver a
new boat from LA to SF once with only working sails. Took four days to sail from Marina Del Rey to Santa Barbara when the
engine died. Actually surfed backwards down the big Pacific Swells and suffered through a
racing fleet sailing from Ventura to one of the Channel Islands easily sailing out and back while we wallowed going nowhere. Let me walk that back a bit, if you have one of the new high aspect ratio rigs designed to work with 'blade' jibs, then a 100% jib can work even in winds below 10k. Still not optimal for light air but enough drive to move the
boat. For the rest of us, an overlapping jib is a necessity
Modern furlers are pretty much bullet proof. Issues in heavier air are usually not enough line on the drum to completely furl the sail and chafe on the reefing line. One of my dockmates sailed down to Palmyra for a little weekend jaunt. His furling line parted after the second day at sea negating the ability to furl the sail. The reefing line was chafing on the
furler case because of a poorly placed lead block. They'd sailed the boat locally for several years that way and hadn't noticed the problem. 48 hours of continuous sailing caused the line to part, however. They had major issues getting the 135% jib down in the strong trades and the working jib was too small to sail later when the
wind dropped. Ended up burning a bunch of
fuel powering for about half the miles down. They didn't want to rehoist the 135% without being able to furl it because of their experience handing the sail earlier. Handling a 135% on a 42' boat was something they weren't willing to have to repeat. They didn't have spare line to replace the parted original furling line so weren't able reconfigure the
furler. Fortunately, the strong trades held on the return trip and the working jib worked okay. Good for them as they were short on
fuel after powering so much on the way down.
If I was serious about not using a headsail furler, would only do it with a double headsail rig. The jib/staysail combination gives more drive in light air than a smallish jib on a
sloop so funxtions over a much wider wind range. Sailed many thousands of miles on our Westsail with only 4 sails, a light overlapping jib, Yankee, staysail and main. The light
Genoa was more of a reacher but gave us some windward ability in light air and enough drive in drifting conditions to keep sailing, had one 15 mile day. since it was made of light material was easy to handle and bage. Dropped that sail going to
weather and went with the Yankee in winds of about 10k or on a reach to 15k. The rest of the time it was Yankee and Staysail or just the staysail. We only made one sail change to go from drifting to
hurricane. The Yankee was always hanked on either stowed in a bag or lashed to the
deck.