Re: shingle roof/metal roof
You should be able to tear off a couple courses of shingles from the cabin, nail your new roof to the existing roof, then install your new roof on the addition. Use a couple layers of 30lb felt, run it up the cabin roof, overlapping the joint between the two roofs. Now take your metal roofing and run it up to the cabin roof and attach it to the new roof. If your metal roofing system manufacturer makes a flashing that will fit the ripples on the new roof, use it. You should use some kind of flashing across the joint. Now go back and re-lay the shingles you removed from the cabin. Lay them so they overlap the new roof. If you see any gaps, you may want to smear some roofing mastic on the joint before covering it with shingles. Any good roofer should be able to do all this for you with a guarantee. Don't know why your contractor can't get it right.