screws from the bottom seems a good idea but it will be hard to get up there because there are a ton of rafters up to criss crossed
I guess we could but why is it different from screws from the roof side ?
No - screws down through the flange into the plywood from above. That DAP roof sealant is good stuff, but it's meant to keep water out, not to really hold something in place. Depending on how wide the flange is you may end up putting some roofing nails through it anyways when you lay down the felt paper (ice paper) and shingles, but a few nails or flat head screws into it to hold it in place would be more then worth it.
Usually when someone says tar in reference to a roof it's what you would find as "roof cement" at your hardware store. It's good stuff and every house should have a bucket of it on hand for patching shingles, flashing, gutters, etc...
http://homeguides.sfgate.com/use-roof-cement-28831.html
