"If frameless packs are generally accepted to be okay up to approximately 25 pounds assuming a virtual frame is created and sufficiently padded shoulder straps are employed, then is a minimal webbing belt sufficient?"
David — to me, a one-inch belt is more than sufficient for a well-packed load of <25 lbs. I've never felt a pinching or any other discomfort. A few years back, I helped convinced Zpacks to reduce the width of their webbing hip belt. After trying it himself, Joe Valesko now offers two hipbelt options: 1" and 1.5".
Whatever the gear, I recommend starting with the simpler/lighter option — and move up to the more complicated/heavier/beefier if things don't work out. More is not always better.