My, my, JyMy! You're certainly a lot bolder than I am on trying to toggle between two versions of OS. Let us know if you have succeeded and if you think it isn't a pain. This is clearly outside my tech skill level.
For what it's worth, iWeb introduced its one and only problem only with El Capitan [OS X 10.11] and still functions normally with Yosemite [OS X 10.10], so for some people the need to be absolutely up to date may not be that important. Then again, the only problem that was introduced with El Capitan can be resolved easily by importing Shapes from another program such as Pages, instead of from the iWeb Insert from the menu bar. Once one accepts that minor compromise, life can be like normal, at least until the next OS X update. Just say "no" ... at least for a while. And do share your frustration with "the folks who know best" at apple.com/feedback/macosx.html - it may not resolve anything, but, hey, it never hurts to try. That's what makes this country great!
Richard