You can nail them down. It dies not come with a "capped" end, just a straight cut. You can cap the ends off with a1/2" PVC end-cap.
Depends what the trellis is built for, but I'd say yes. As to the painting, ALL of the rods available at the store I visited were ALREADY rusted. You'd have to REMOVE the rust, THEN paint.
I cut my rebar to length with a cutoff wheel on my grinder
About 5 lbs
This is from Weyerhauser's SDS at https://www.weyerhaeuser.com/application/files/9715/3806/4519/Steel_Rebar.pdf "When this product is welded, cut or ground, fumes are generated. Welding fumes may be different in composition from the original welding product, with the chief component being ordinary oxides of the metal being welded. Chronic health effects have been associated with the fumes and dusts of individual component metals, and welding fumes as a general category have been listed by IARC as a carcinogen (Group 2B). Some welding fumes contain known reproductive toxins, e.g., lead, which may be present in the coating material of this product. " So, yes, with appropriate precautions
Rebar would be the best choice along with fiber in the concrete
They don't have have anything set up at this time to cut them.
probably made in china like most other metal products and other home depot products
They don't have anything set up to cut them at this time.