If you watch the seedheads, they will turn brown and you will know that the seeds are ripening when the Goldfinches cling to the heads and start picking off seeds. They eat them in tidy little rows. I try to harvest some of the seedheads before the finches get them all, and store them in the garage. Then I tape up a bunch of them to one of the bird-feeding poles for the Finches to eat during the winter (they love 'em). Despite the finches eating most of the seed, the Echinacea still self-sows abundantly in the gardens, too much even.

No clematis do not need to be deadheaded. I generally do it on young plants just so that any seeds that develop get removed so the plant can concentrate its energy on its own development rather than the seeds. However, there is no need to do so on most clematis once established. This is especially true if you want to grow clematis from seed--where would you get seeds if you deadheaded them all?

I don't do much deadheading on mine...too many of them, and too many weeds to pull :) However, if some of the blooms get particularly nasty looking, I cut them off, just for the sake of appearance. I've not noticed that leaving the seed pods in place has had any ill effect on next year's bloom, or the growth of the plant.

It doesn't look bad for a newly planted clematis though if it was previously in an area with fewer brightly reflective surfaces, it may be dropping leaves that aren't able to adjust to the new brightness. Did you plant a bit deeper than it was in the pot? A clematis is one of the few plants that like this treatment as it encourages new stems to grow from below ground, so you may see new stems at the plant's base. Do you know what kind it is? Usually it's suggested that new clem plants get cut back to a few leaf nodes per stem, which will encourage growth of more stems and a denser growth pattern. Some will come from below ground, some from the top of vines, and some from secondary stems at previous leaf nodes, especially if the top of the vine has been pruned. Yours looks like perhaps the vine was cut or is that just a product of the photo? No need to specifically prune off the seeds, which IMO are ornamental in their own right, though it isn't a problem to do so. Type 2 prune plants regardless of age do well with a light prune after bloom since it encourages new growth and late summer or fall rebloom in areas with a longer growing season than where I am. IME clems usually put a lot of growth into roots the first season, though it will vary some depending on how large it was to begin with. I plant smaller clematis since most of mine are mail order, and I really start to see a lot of above ground growth the second year, though they do grow some leaves and vines the first year. It would be far happier with an organic mulch such as bark mulch or compost than rocks. The combination of cement pool deck and rocks can probably get hot, and if there's landscape cloth (which often gets put under rock), that will not help since it tends to interfere with movement of water and nutrients. A wire tellis will be easier for the clematis to climb than the wide trellis pieces since it climbs by wrapping its leaf stems (petioles), so does best with something pencil thickness or less. You may have to periodically fasten the clematis to the trellis.