Brilliant Tweetie. Really appreciate your work.
I have only a rough grasp of this so obviously will defer to Ken and RWM, but I believe the Altocumulus layer earlier this morning sat (just) in the -10°C to -20°C (roughly) zone that favours the growth of branched ice crystals that start snow flakes. I believe it is referred to as the dendrite growth zone.
Looking at your first pic it appears to be 'snow' virga, we often go through large flakes around 10 to 12000ft in virga like this. Even though the Altocu bases are much higher than that, your Virga looks to have made it down to maybe 12-13000ft?
It is even 'thick' enough to have Mammatus features.