Happy to be answering, not asking. Here goes:
Shingles are absolutely CRAP on the edge. Wind will blow up under them, rain (or even dew) will wick up under them, etc. The metal edge you see on every properly constructed roof is called "flashing" - its whole reason for existence is to mitigate those issues. Flashing has to be laid in a particular way - at the upper edge of the roof, it goes on top. At the bottom edge, its right above the decking. The idea is that you want to give water a path that is always "down". There is some dispute about the sides - its usually above the felt, below the shingles, with the top edge below the top flashing, and the bottom edge above the bottom flashing.
Underlay is whatever you place between the shingles and the decking. It used to be 30# felt, typically. Or two layers of 15# felt. Basically, its a paper like product saturated with a tar like product to repel water, which has some minimal self sealing properties to "grab" around the nail penetrations, so the ~500 holes per 100 square foot you just punched in your decking while nailing down your shingles don't serve as paths for water to penetrate your roof. Generally, one thick layer is better than two thin layer (contra expectations), as every wrinkle, deformity, etc in the decking or the felt will be magnified in the appearance of the shingles on top. Think wrinkles in your sheets, or your comforter. Thinner material rips more easily, shows more defects, etc. Honestly, I think the only reason 15# exists is so that people could half ass a re-shingle job, when the budget didn't exist to do the job correctly.
In the case of shallow roof designs (often porches and the like, basically anything with a pitch under 3/12), water doesn't run off felt underlayment very well. Where the decking underneath sagged (such as between rafters/joists) water that got past the shingles could pool and sit, eventually making its way down the nails and rotting out the roof decking. Various manufacturers (I'm fond of GAF) made modified bitumen underlayments - they are basically a reinforced rubberized material, frequently self sticking. MUCH better at repelling water, much better at repairing holes from nails, better with thermal expansion/contraction and moisture accumulation from freeze/thaw cycles, etc - but also considerably more expensive. These only recently became available to homeowners, a few decades or so - but used to be the way they made flat commercial roof (absent the self adhering properties) Based on your drawings, you have more than adequate roof pitch, and don't need to use this option.
Hope that helps, and I hope you can convince the powers that be to forego the shingles. If you DO go shingles, snap chalk lines - do NOT rely on the lines printed on your plywood decking or try to eyeball it from marks on the shingles themselves. Trust me, you will be happier with the end result.
Oh, and you DO NOT need to buy a special start course. You can. OR, you can turn your shingles around, and cut off the tabs with a sharp craft knife, use the "half" (actually, a bit more than that) shingle you have then created as your starter course. The first full shingles will then go on top, with a 1/4" overhang.
sealed the seams with outdoor silicone too. You can see the texture behind the boy in the box. Lol. 
