How to remove www from all requests so wildcard SSL for *.exmaple.com will work for www.client1.example.com?

You can only do it for HTTP, because in HTTPS, it is already to late (the certificate error is already there). Try this solution from chomps:

You’d put this in www-backend. However I have not tested this personally and I am not sure at this point if the redirect really considers the already rewritten host header. Try it.