The sounding below may explain some of the behavior we see here. The steep lapse rates above the frontal inversion at 600 mb may be the level of the castellanus. The slightly lower altostratus deck moves in at a slightly lower level, perhaps near 700 mb where the air is stable even when saturated. Then the nearly stationary clouds may have occupied a layer where the sounding showed light winds at 800 mb. All this is really conjecture at this point but it's fun to guess what's happening.
For an excellent discussion on Castellanus, take a look at Corfidi et al. 2008.
Corfidi, Stephen F., Sarah J. Corfidi, David M. Schultz, 2008: Elevated Convection and Castellanus: Ambiguities, Significance, and Questions. Wea. Forecasting, 23, 1280–1303.