Unless mistake (I'll re-test on monday), I think I found the right way to achieve the goal: force both SSL and www whatever the scheme, knowing Webuzo doesn't allow to separate www and non-www in two server blocks. It seems http://domain.com,http://www.domain.com and https://domain.com (the most problematic one) ) are all redirected to https://www.domain.com now. Apparently no infinite loop this way.
I added an extra conf for domain.com from Webuzo > Domain > Extra Configuration with this content: