1- Regarding the Wisteria, I bought 2 plants last spring (About 3' tall, 3 gallons). They started to climb very quickly. I would have bougth 4 of them, but the other 2 posts are on the concrete deck.
They have climbed very fast to the 12' high structure. I had to help it a bit direct its growth in the proper direction. If I would do it again, I would prune/support it more during the summer while it is growning and focus on a few major limbs for the climbing. I will try to clean it up this winter.
2- It covered about half the structure by mid summer. I expect it will fully cover it this summer (2nd season). That is what I hope. I have seen very large areas covered by a single vine, the trunk can get quite big. Be sure to put big posts (e.g. 6"x6")
3- Regarding the staining, I did not do it before assembling the structure, and now I have some regrets. It still has a nice color, but much lighter red. I expect it to be gray by the end of next summer. I kind of like the redwood color. By the way, this wood is incredible (sorry for the living tree). It smells great and it is very easy to work with. Definitly worth the extra price. Try to find a good lumber supplier that specializes in Redwood/Cedar. If you are in DFW area, try Lee Roy Jordan, I was quite pleased with them.
4- Generally, the soil in my area is heavy clay. However, the area where I planted the wisteria is mostly sand from the deck/pool building. So the drainage was good (not sure if important for wisteria), contrary to where I planted my dying RedOak
, but that is another story. I imagine the root system will get to the clay, if not already.