I'm going to build a wheel with 650B rim (VO Diagonale) and a front hub with unusually close together flanges (SP dynohub, center to flange only 22 mm). Should I worry about the spoke triangulation angle?
No. The drive side of rear wheels is worse and works fine.
Should I build to higher tension to compensate?.
No. Wheel tension has no effect on stiffness.
Any reason to prefer 2 vs 3 cross?. Spokes will be butted, like DT Competitions.
Number of holes, spoke bed diameter, and hub flange diameter - you want the spokes as close to tangential as possible for strength, and don't want to overlap the heads which would complicate replacement.
As a roadie I've only dealt with 700C / 622 rims and lack practical experience on where smaller rims like 650B / 584 need fewer crosses.