The first method does not work for me on HDP 2.6.2. What happens, is that the SOLR UI accessed through Knox (Or Knox itself does?) forwards to the actual address of the SOLR UI, and obviously that access doesn't work (due to firewall, differing Kerberos realms - whichever of the reasons you use Knox for).
Are you aware of any supplemental configuration to achieve a working Knox-proxy with the HDP-Search-SOLR?
Adding /solr to the url helps marginally, because it avoids the redirect, but the URLs and content are not working correctly.