If you make a fake gutter box, you will have to extend the roofing over the box, whether you use shingles or metal. In either case you will have to tie the new roofing up under the existing roofing. Here are a few products that may help, whether or not you decide to add gutters. They are alternatives to soffit vents.
Cor-A-Vent goes under the fascia: http://www.cor-a-vent.com/s400.htm
Drip edge vents replace the drip edges. You mentioned that they get mangled by ladders, which is true unless you add gutters under them. See below: http://www.certainteed.com/pro/ventilation/products/intakevents.html http://www.oikos.com/esb/50/globe.html http://www.tamko.com/res/rollvent/pdf/997655_7.pdf
You are right -- waterproofing the seam is tricky. You have to take back the roofing material and add a waterproofing membrane or ice dam shield membrane. By the way, if you're going to extend the roofline then it's easy to add soffit ventilation...