PVC parts do NOT off gas chlorine. At least not by the time the extrusion process is over. Any excess chlorine is long gone before the gutters get to you. The only issue with off gassing or problems will come if the gutter is burned or exposed to high heat (400F+) for significant time periods. In that case you don't get chlorine but, HCl or Hydro-Chloric Acid as a gas. Some of you with a pool will know this as Muriatic acid, but in a much stronger and pure state. That's what happens if you have PVC covered insulated wiring in your house and it burns down. the Fumes are toxic. But only if it BURNS. This is from a Plastics Engineer (MS Eng with 35 years of experience. (and a chemistry degree)
With the White Vinyl K-Style Joiner, https://www.homedepot.com/p/Amerimax-Home-Products-White-Vinyl-K-Style-Joiner-M0608/100087674 See the assembly instructions here https://images.homedepot-static.com/catalog/pdfImages/29/298efeac-47c5-426a-bc28-6f7a9ecfe308.pdf
"Zinsser 123" primer will stick to almost anything. Talk to a paint pro first about plastic, or read the product info on the can. I've used it on lots of unusual surfaces and regular paint goes on and stays on. That said, plastic gutters are a poor substitute for metal. Aluminum is well worth the money.
Usually a 1/4" inch per foot, sloping toward the downspout.
On a good day, 10-ft = 120-in
Go with 11 pieces of gutter seeing how a joiner costs more than one piece of gutter so if you try to splice the pieces you end up spending more money. Gutter manufactures make their money charging for the pieces to hold the gutter in place than on the gutter itself.
Heat is not the factor here. Sunlight (UV) is what breaks down plastic. They should last you over 6 years.
I don't know, but I am replacing the screw with stronger less expensive screws...We are just now getting them up....
No printed guide is provided, but it is pretty straight forward. Measure where it needs to be installed, including the location in which you want the downspout. Cut to size, measure the pieces preferably assembled, and do adjustments on the ground before taking it apart again for glueing it using silicone (waterproof), and reassembling it. End caps for the ends, Joints for the joints, you have the downspout for where ever it is needed. After that you just need to install it, using gutter hooks made for the gutters, spaced out a foot or foot and half (depending on how strong you need it to be), through the area you want to install.