Tree Stumps Advice

Drill holes into the stumps and put in rock salt. Then pour over hot water

I would be tempted to cut across and down, plane, sand and varnish to use as seating.

You could use Roundup Tree Stump - I know that one works. Epsom salts work too but they all take a bit of time and you have to also allow for the sinkage in your ground when the root systems shrivels and collapses. This article may be of use. https://www.rhs.org.uk/advice/profile?pid=229