Yes, it can be grown in a container - I have several clematis in pots growing on my deck. How large a pot depends a good deal on the type of clematis - some clems are bred specifically for this purpose and are quite compact plants. But other clematis can get to be large vines and will develop a root system to match. These need a generously sized container to accommodate their larger root system. My larger vines are in containers that are about 24" deep and about 18" across at the top with a slight slope to the base - like an oversized flower pot. You want a good textural, fast draining potting mix and add some CRF like Osmocote. In zone 7b, there is no reason to assume this container could not stay outside all winter. The large flowered hybrids are hardy to z4, so quite able to withstand the cold if in a generously sized container and z7 soils seldom freeze to any appreciable degree anyway. It would be more a matter of the durability of the container - if you use a plastic or plastic resin pot or a freeze proof glazed ceramic container, you are good to go!

Pineapple is easy. Just use any three to five gallon container. Whatever you like. The first one I grew is two years old. It's in a five gallon container with three mini roses. Its three foot wide and at least a foot tall. We only get one month of summer - August so it's starting to get a growth spurt as the weather hits 80. Hoping for my first homegrown pineapple this year. I have a bunch of first year ones in three gallon containers and they're getting more sunlight and double the size in a few months. Just bought a couple and placed them in a 20 gallon container with herbs and strawberry.

Plastic lined whiskey barrels work nicely. I gave up all my waterbowls because raccoons kept trashing them :( too small and they could tip them over. Waterproof foam pots with no holes work too and they are often cheaper than stoneware/ceramic. I've had a lot of ceramic pots crack in winter over the years.

I think that what "look" you are going for will drive both the container and furniture choices. Right now you have a limited palette dominated by green, with texture creating the interest. If you want to continue that calm feel and the outward forcus toward the river, then any planters should continue the palette of green, white or gold (not sure what color the hostas have) with the plants and gray/black with the furniture and the pots themselves (if any) also in that range. If your goal is to add more energy to the scene with the perennial plantings providing the frame, then choose furniture items and pots that include other colors that really pop. I don't think I would add just a little color or pastels; I would want one of the extremes. For the second scenario, the pots would be needed, but for the first, more limited palette, it would depend on the furniture chosen, both style and amount. You don't want that arrangement to look cluttered or busy. I know that's you didn't ask about this, but I find the tall things by the river to be disruptive to the view. Can whatever function they serve be accomplished with something shorter or something mounted in the trees?