I am sorry you have to go through this painful exercise. But, how would an alternative rendering engine fix this problem?
When people want to get their work done, opening another browser while the original one is being fixed (which might take a couple weeks), for certain applications, is extremely reasonable for business people just trying to get their work done.
This foundation is provided by browsers and is also broken by browsers. If safari's foundation is broken then safari is the one not fulfilling that fundamental promise, not the websites now broken. So yes, don't use a browser if it doesn't work.
Before Opera gave up and became another Chrome reskin, there were plenty of sites that didn't work for me until I set the user agent to match another browser.