You need to ammend your soil. Carefully dig out the clematis and put it aside in a bucket while you fix 'its' home. Don't let the roots dry out. Add compost and sphagnum peat to a much larger area than just the planting hole. Dig out the soil and add the ammedments to the soil you dug out. 8 to 10 inches down should be more than sufficent and make the area nice and wide. Mix well and then add some back into the dug out area. Water, plant your clematis back into its nice new home and finish adding the soil back. Water well.

You can use all kinds of perennials to shade the roots. They should be well behaved, not invasive or too agressive with their roots. They don't have to be extremely early in leafing out because it is usually cooler in the spring and the clematis shouldn't have many opportunities to over heat. You could try Heuchera, Lady's Mantle, Lamiastrum, Pulmonaria, to name just a few. Their roots system shouldn't be a problem for your clematis.

I have never experience clematis wilt or even familar with it so I don't have much to say about that, sorry. I have never needed to feed my clematis but I do keep the soil mulched in my perennial beds which over the years replenishes the soil. It is a continual process.

Happy Gardening,
Diane