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.

If you do plant it in a pot, I would only do so for one year before putting it in the ground with plenty of bonemeal and composted manure, about 2 cups of each mixed well and placed in the bottom of the whole. Then I would buy another clematis next year to plant with it. Clematis plants become very addicting, In another season or two when it is covered with flowers you will understand and to see a combo of another clematis planted with it or a rosebush, especially a climber is breathtaking. Put them on a trellis or fence or something to climb up. If you do order another one, get it through a much better place like Silver Star Vinery or Chalk Hill. I have dealt with that company too and choose to stay away from their tiny little plants,especially clematis. Look up daves garden on the internet, go to garden watchdog and there you will find ratings and writeups on how these mail order companies due and who to stay away from.

What you feed is dependent on your soil type, i.e. if you have clay, rocky, rich, etc... how fast your soil drains and what minerals make up your soil. i.e. is it high or low pH, calcium/magnesium based... generally you can tell your pH level by your water hardness, soft water means low pH, hard water means high pH. You can always add organic manure, mulch, peat moss etc... you're only going to have 1 shot at preparing that bed. I always toss in lime (calcium/magnesium grit) in mine because I'm in the North East, and a healthy dose of the cheapest 12-12-12 fertilizer when making a bed. Then each season I mulch and a couple times a season I add the slow release fertilizer in pelletized form because it's hard to burn with those granules. I'm pretty new to clems, but from readings, where they do best (i.e. england, northwest US etc...) they like a rich soil (high organic and nutrient) and plenty of water, as long as it drains. So fertilize away as long as you have good drainage, if you have clay, dig down with organics as far as you can, even just a bag of mulch mixed in to increase the organic matter can help (although not too much non-decomposed matter as it can rob nitrogen initially).

I think I would leave her alone. My brother has two clematis together and they do not grow very tall at all, but what a show they put on. Dawn