By far the best canvas signing pen that I have tried (and I've tried quite a few) is the "Pentel Sunburst Metallic Medium Gel Pen" in the silver color. Writes easily and smoothly on uncoated, printed canvas and does not clog, smear, or randomly deposit puddles of ink. Claimed to be acid-free, fadeproof, etc. Silvery but not obtrusive, keeps a consistent fine line like an 05 Micron pen. I'm up to around 40 prints on the same pen that's about 5 times better than the previous record holder. BTW the worst performer ever are the DecoColor pens...puddles galore look out.
Don't know about certificates and such, but I mount my canvases on hard substrates with a few handwritten words on the back such as title, date, location, "To Jack & Jill!" maybe even a short comment. I have stopped editioning new work, but for the first few prints I sometimes write "this is the 4th print of this image" or whatever. Buyers love all that stuff, especially at art fairs. Also sign on the back as well.