I rode again extensively trying to look for the shimmy that Jipe mentioned, but there was none. Maybe the pack has at this moment too much load for that. The width is ~63cm. I'll be riding to work, of all places, tomorrow, presumably with a bit less load, so will be trying again to see whether I can run into any instability.

The beauty of the current mounting is that attaching the pack to the bike is as easy of not easier than a T-bag. There is less interference with the handlebars and gear there than for a loaded T-bag. The issue of those extra thin straps, suggested by Winfried, is the same as of those two extra attachment points I was mentioning. Maybe, again, the problem surfaces for a less loaded pack. For the moment, I will just practice riding with the pack and see whether any issues come up. Somewhere farther down the road I am think about adding extra curvature to the S-bag frame.