I think the problem with the sleeve is this:
Consider point B as merely a starting reference point, not the actual point along the sleevecap seamline.
Use the Arc tool to create an arc radius=shoulder point and underarm length, and center is point B3, from angle 0 to angle 90 (you can narrow this arc up later to have less 'sweep' if you wish).
Use tool Intersect Arc and Line to intersect your arc with Line_B_B2.
The intersection point B5 will be above (or below) point B along Line_B_B2.
B5 is now the actual height of the sleevecap.
If you know about sleeve caps, then you know the wider the bicep line (Line_B2_B3) the shorter the sleevecap (Line_B_B2).
The position of B3 is determined by your sleeve width at the bicep line.
The position of B5 adjusts to the width of the bicep line and the length of shouldertip to armpit in the bodice.
For tight sleeves, Line_B2_B3 is narrow and B5 is high. For loose sleeves, Line_B2_B3 is wide and B5 is low.
So by using B3 as the point to determine the actual height of the sleevecap, you will always have a sleeve that works.