I agree with you that stays seem to be needed in many cases. That's why I tried this pack out rather cautiously at first: to see how it went with a good load but no stays.
Technically, it seems to me that what matters is not the stays themselves, but the rigidity of the pack when filled up. Does the shape slump? This was well covered by Ryan J in his two articles on the structure and load carrying of frameless packs:
The first trip, in the Australian Alps, suggested that the pack was stiff enough with just the foam frame sheet and no stays. The European trip confirmed this for me, at least up to about 12 kg load.
Yes, the split hipbelt worked fine.