Yes. The Oatey High-Rise Thermoplastic All-Flash No-Caulk Flashing is designed for roof up to a 60° angle (20/12 roof). 
No. The Master Flash and Retro-Master Flash are specialty flashings designed for engineered roofs.
No. The Oatey Master Flash or Retro-Master Flash should be used in this application.
This question doesn’t have a correct answer because of many variables that could affect the life span of a flashing. Examples are extreme heat, extreme cold, and hail.
Oatey Master Flash and Retro-Master Flash are rated for continuous temperatures up to 212°F. All other Oatey flashings are rated for B venting, which are continuous temperatures up to 180°F.
100% exterior or marine rated silicone caulk can be used to seal plastic base flashings to the rooftop.
Single wall metal exhaust pipe should never be used to vent a water heater or furnace when the pipe passes through a combustible material of any kind due to risk of fire. The flashing rain collar will melt or warp if exposed to these temperatures passing through the single wall pipe, and loose the water tight seal it is designed for. All water heaters and furnaces not listed as high efficient should be vented using rated B vent pipe. B vent pipe is doubled wall pipe which isolates most of the exhaust heat to the interior pipe. All Oatey flashings are safe to use with B rated vent piping.
With the exception of the Master Flash and Retro-Master Flash, all flashings have a one year warranty from the date of manufacture. Flashings cannot be guaranteed for the life of the roof due to varying roof life spans, and potential adverse conditions (i.e. extreme heat, extreme cold, hail). If a roof is replaced the flashing should be replaced at the same time. Master Flash and Retro-Master Flash are the only exceptions; if installed properly according   to industry standards, they have a 20 year warranty from date of manufacture.
Yes. The Oatey Master Flash and Retro-Master Flash are designed to conform to most roof profiles.  
After the correctly sized collar has been selected; you will apply a 100% exterior or marine rated silicone caulk to the top of the existing rain collar attached to the roof flashing, and also to the underside of the new rain collar. Slide the new rain collar down over the pipe to the top of the existing collar. Press the new collar down with enough pressure to seal the two together without collapsing the seal around the pipe.