Sorry I have been a little tardy in getting back to you Clyde I have been on holiday and needed to get back to my machine to check out this problem. Firstly you are educating me a little bit here - I had no idea that there is a key stroke equivalent for the double click.
I am using Excel 2010 and the latest version of XLC Ribbon V13.09.01 and I have a similar problem. The double click 'sticky' paint button is indeed lost when XLC is installed - it functions correct on a single click but a double click behaves as a single click.
Might I suggest that you continue using the work around you suggest and temporarily disable the XLC addin when you wish to use the double-click 'sticky' paint button. I'll have a think about if I can offer any better solutions. Thankfully I don't think many buttons have double-click 'sticky' functionality of the paint button - so I hope the issue is limited to the paint button.
Thanks for reporting this in Clyde