Trench means dig a trench between your garden and the black walnut. Cut all the roots you can find. An axe and a handsaw will be handy, if the soil is wet you can cut through soil with a handsaw. Small roots can be cut with the spade directly. That will leave some dead roots under your garden, but good microbial activity is all you need to decay the juglone to low levels. In my previous garden I would double dig every second year, cut every root (these were maple roots), and backfill. With a BW, maybe every year. The BW is in the front yard. I tried to plant first tomatoes (instant death), then garlic and squash. The latter were really small, so resistant is a relative term and you are forced to trench if you want a crop. Juglone weakens all plants. Most herbs survived, except for thyme which is smaller every year, so right now in front I have all manner of herbs and flowers. Sage, parsley, sorrel, mint, oregano, marjoram, lemon balm, nettles, fennel can all take the juglone. Chives does not grow but does not die, and basil does not like that area at all.

I doubt you killed your clematis, but you may have cracked the fragile stems at the base. Keep treating the roots like they are alive and you will probably get new shoots coming up. Best practice is to prune the clematis back hard before you plant leaving only about 6 inches of growth or taller so you have a few sets of leaves. There isn't a leader vine. Sweet Autumn grows fast, so keep watching it so you can try to guide the stems. I add bone meal the way Chalk Hill recommends, but it isn't required. I don't take out the supports that came in the nursery pot unless they fall off. I try to cause as little disruption to the roots as possible. I do take off the twist ties though and use my own. Tightly tying the vine to your support isn't good. I use velcro straps to hold the vines in place. It doesn't take long for the tendrils to grab on, so as soon as it does, the straps can be removed. What I try to do when planting a new clematis is plant it at an angle towards the support. That way the vines will already be growing in the right direction. To help it along, I might add a couple of lengths of thin bamboo on either side of the root mass and use these to help guide the vine. It also serves to protect the vine (somewhat) from dog feet. Where practical, I will use the velcro pieces to hold the bamboo to the support, but most of the time it can just rest on the support because the soil holds it in place.

I've had volunteer squash try to grow in unprepared clay soil and become stunted. Is your top soil a pretty good depth? The leaves on the plant look healthy but kind of light... So the plant may just need to be fed.. It don't know if it's just the picture though.

Normal result from a tip-pinch only, 1 or 2 new tips but no real reduction in upward mobility or increase in girth. Remove the lower leaves to encourage lower branches at the nodes where those leaves were attached. Practicing on Coleus is so fun because they grow so fast.