Draft split on curved faces.

In general, I used the surface representing the parting line to 'split' feature the solid geometry into "a" and "b" halves, and then used the parting line type draft feature to add the draft relative to plane1. Had to move some of the filleting to "after" the split and draft work to get it to flow correctly, and suppressed a bunch of stuff that was making life difficult for the parting line drafts. Here's a screenshot of the draft check set at 1 degree, and I uploaded your file with my tweaks. Hope this gets you close to what your after!

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