the metal is good if you cant flat bar out that under layer of shingles without a problem.i use this method alot out my way where a low slope epdm or pvc roof meets a slate roof.very simular to your situation,just w/ slate instead of shingle.i dont trust that cap sheet your using without somekind of subroof or underlayment.
Regarding to get at least 18" under the shingles… That will be addressed when I replace the shingled roofing next fall. In the meantime, I want to mention that I only get 8 (or less) inches of rain per year. The sun and the wind are my enemies until then.
That said, anyone have ideas on how can I complete the job without tearing into the shingled part of the sloped roof?
im not trying to break your balls but looking @ your picture your nails in the under lap are real close to the cover lap. id go to the top & use button cap nails, simplexes, or tincaps & nails. just hate to see it blow off into your yard.