Radiant barrier in the attic is an excellent idea. At the typical roof temperatures you see in the summer, even in a temperate climate, radiant transport of heat is significant and a properly installed radiant barrier can be very effective in blocking or reducing radiant transport of heat.
California Title 24 now requires some kind of radiant barrier. My house was built just before this requirement took effect but I elected to pay the small premium to use TechShield radiant barrier anyway and it has been worth it. I reviewed the AtticFoil site and they seem to have a good product for retrofit applications. Your plan sounds like a good idea and I don't see any major issues aside from the usual ones of "work carefully, try to avoid falling through your ceiling, prepare for the high temperatures up there, watch out for dust etc".
Do try to stay away from the "over-insulation" method as that loses effectiveness as dust builds up on the top (working side) of the barrier. Your plan to use the "open-ridge" method is the best as dust buildup on the bottom (working side) of the barrier is not as much of a problem.
Insulation of the ducts is not a binary yes/no question any more than attic insulation is. funkadelic understands this and that is why he plans to increase his attic insulation from R19 to R50.
Yet duct insulation is typically no more than R4 to R8 while carrying the coldest air in the house. It is the part of the HVAC system that has the least insulation while needing it the most.
No doubt there are some installers who think that as long as "cold" air comes out the register there is no problem but this ignores the question of how cold is the air at the end of the duct vs the air that went into the duct and how much cooling power has been lost in the duct.
Who would accept a water pipe that leaked half of the water it carried?
The Federal government has done many studies of ways to reduce duct loss because it does make a difference in your energy efficiency. There are even suggestions to move the HVAC ducts into conditioned space as this would cut duct heat/cooling losses to zero. I did this in my house.
For retrofit, moving ducts is impractical so the next best is to bury the ducts. You have an excellent opportunity to do this when you upgrade to R-50 of cellulose.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ns/eemtg082011_a8_duct_retrofit.pdf
You do not want a vapor barrier on the cold side of the insulation. Switch your plan to use an air barrier (or an open mesh, since you should have little air movement in the space) and it makes more sense. On the other hand, if you already bought the plastic, just poke it full of holes (but do not poke through to the warm-side vapor barrier.)
But - this will leave you with essentially uninsulated areas at each stud, and compressed insulation near them - particularly galling since they are "flat-ways" (1-1/2" thick.) It would be better to attach ~6" wide boards to the studs (sticking out from the wall), so you have a nearly 6" deep cavity to fill with insulation, and no compression where the plastic or mesh is attached.
Edit post-comments: You could also fill in between the flat studs with 1-1/2" rigid foam cut to fit (and/or sprayfoamed at the edges), and then sheet over the whole thing with 1-1/2 to 2" foam covering the studs (with a bit more sprayfoam sealing any joints or gaps), but your question indicates that you already purchased materials, so I don't want to send you off on a mission to spend more money if it's not really needed. But if you'd rather do that than add boards, it should work fine - especially if you can return the materials you bought.
Best Answer
Attach to the stud face. The vapor barrier is not as effective if stapled to the stud sides. And as you say, the fill won't be ideal. (are your external walls 2x4 or 2x6?) The paper on the fiberglass is the vapor barrier.
And a barrier is definitely needed where you live.Optionally, you can use unfaced insulation and a plastic sheet vapor barrier. I've done lots of sheetrock over insulation and the insulation is not the problem with popped screws. It can cover and hide forgotten screws/nails/bumps which will cause problems. One bonus is when there is a couple layers of paper between the sheetrock and studs, it can cut down on noise/vibration conduction through the walls.