Re: Insulating sizzle joist ceiling
You can always run some framing (even something like furring strips might work) between the trusses to act as fences and help keep the cellulose from sliding. This would also give you the option of running your steel parallel with the trusses instead of perpendicular to them. Personally, I think it looks better and unlike running the metal perpendicular to the trusses, it will allow you to have single sheets of steel that run from the eaves to the ridge with no seams.
Is 12/4 the roof pitch, or the pitch of the bottom chord of the truss? The bottom chord will have less pitch than the roof.
Also worth noting that things get pretty tight at the ends of a scissor truss. If you're planning something like 12+ inches of cellulose (R 38-ish), you'll find that the last couple of feet near the eaves, you'll probably have less than 12 inches of space between the bottom of the truss and the roof deck. So, as long as you have proper baffles in place to keep the cellulose from filling the soffits and blocking any vents, you could probably fill the truss bays with cellulose and it wouldn't really have anywhere to slide to. Here's a decent pic to illustrate how the eave ends of scissor trusses will have less space for the insulation to fill:
