That trellis could get too weighted down at some point but you could add some posts to shore it up. The advice on kiwi trellis is to build them like a tank, I built cheap ones and with a few minor mods have kept things working fine for about 20 years.
Re twisting around the wires, yes you definitely want to unwrap those; you can wait til winter though as it is easier on a dormant vine. Also kiwis will later make stronger straight shoots from the base and you may want to replace these first ones with new shoots.
You can put several cordons on, but if things overlap too much you may want to remove some eventually. The main point is to not get growth too thick overall.
For the grape just train it up a post about 5’ off the ground and let it get a shaggy hair look from there. Prune back to the post in the winter with only stubs of shoots left. This is an ancient pruning style you can still find a few places (coppicing is the name I think). You don’t want vines on the ground they will get eaten by all sorts of pests.