On your plants in the shallow planter, well big plants that need lots of water and nutrition might make it to maturity in there, but you are in for a lot of work to keep them going. They just can't grow a big enough root system to feed all the growth that the sun and warm weather are stimulating. So . . . the answer is "maybe".
Your Coleus look amazing! Yes, you will be able to keep cuttings going on a windowsill indoors through the winter, I would think. I have cuttings of Coleus that have grown and flowered in a glass of water. Personally, I'd forget the idea of collecting seeds and just propagate them from cuttings. You can put 5 or 6 cuttings in a small jar of water. Seedlings take a lot of care and attention, too.
Your pumpkins are really pretty. Traditionally, pumpkins get the most sugar into the flesh if you leave them on the vine until the vine dies. But if you are only going to use them for decoration, you can pick them now and get rid of the powdery vines. You're right, that powdery mildew will spread to any other pumpkin, squash, melon or cucumber plant nearby. They are very susceptible. Housekeeping (taking off any leaves that show the fungus) is the best way to keep the other plants clean.
You can also spray a solution of 1/2tsp. baking soda to a liter of water to prevent the spread of that fungus. Many plants aren't receptive to it anyway, but the ones that are, you can make the surface of the leaves not welcome the spores with this solution.