It is advisable to plant a clematis about 3 to 4 inches below ground level but we don't always do this. Having said, cut the clematis down to ground level and use a diluted fungicide to drench the local area. Hopefully, the clematis will re-shoot. If this doesn't work, I wouldn't try and replace in the same position.

Yes, do as above. Also try to clear all debris from around the plant and give it a good mulch.

I find clematis wilt does tend to stop after 2-4 years, I now have about 10 and over the years I have planted them deeper and deeper mulching and watering a lot that first year. I use to cut the 2 with clematis wilt down to the ground and give them a dose of Epsom salts twice during the summer months.

Wilt is a lot rarer than people suppose. More often than not when a Clematis starts to droop it is because of stem damage. Snails love to chew on them at or just above ground level.
Remember Wilt begins at the bottom of the stem and works up, mollusc damage begins at the top of the stem and works down. Also with wilt the stems have a purple stain inside.

I heard a tip from a castle gardener once re planting clems. He said plant through a short section of one of those old fashioned clay drain pipes and that this keeps the problem infection from reaching the stem.