Not to drag this on forever, but it may be useful for future Seagull Spacewalker II newbies. Further sim experimentation -- this time with greatly exaggerated throws and the radio set for no expo on ailerons or elevator along with slow speed flight (about 1/2 throttle) -- also yielded a "wild ride" similar to that of a badly tail-heavy plane. So given I've checked and rechecked the CG inverted and right-side up, it seems like this second scenario is the most likely cause.

Accordingly, I've set the throws to the recommended minimal throws on low (1) and to the aerobatic throws on high (2) for both aileron and elevator. I've also set the expo at 20% for both aileron and elevator.

Once I fly it again I provide a further follow up report.