Have a overhang / leantoo built off the garage that was never completed and covered with a tarp. I've stripped the rotten plywood off and have new OSB to install. The slope is 5" in a 12' run. A fraction more than 1/4" per 1'. My thought is to install a layer of felt paper then rolled asphalt nailed at the top edge and adhesive along the bottom??
I don't think what you are proposing will work. Asphalt rolled roofing is NOT waterproof. Sealing at the joints and eave will stop wind blown rain, but standing water will saturate the paper and it will leak. (The paper is not UV resistant either.)
I wouldn't use a "peel and stick" membrane either, like Grace "Ice and Water Shield", because they can only be used on slopes down to 2:12. (...plus they have no UV protection.)
I'd use an PVC membrane. I would not use an EPDM membrane. The main difference is that EPDM's seams are chemically sealed and PVC seams are heat welded together. The EPDM seams can breakdown over time.
...but back to your question, No, I would not use roll roofing sealed with mastic.
I'm guessing that the reason your not going to torch it down is your uncomfortable with fire? If so then I would contact a roofing supply and get a two part roll roofing system that has a nail base. It essentially has a sheet you nail down then a sheet you glue down ontop or some systems even include a peel off backing to be applied to the nailed base. And are very easy to apply if instructions are followed correctly. Oh and minimal slope for most rolled roofing products is 1/12