So sorry to learn this.
There is no other way that I know of to revert to the older version of FaceTime because Apple doesn't leave the older versions of their apps up on the Mac App Store servers. (They do this same dumb thing with iOS upgrades and apps).
You need to submit feedback/bug report.
Submit this under the feedback for your type of Mac as there is no way to submit feedback for just FaceTime only or based on eartlier OS X versions.
This is why having proper backups of your entire system is prudent and can come in handy.
You should have a conputer back plan, anyway.
Everyone using a computer should have system/data backup plan.
Just my opinion, but I, absolutely, dislike Apple's Time Machine app for backups. Do not like Time Machine, at all.
It has failed me more than a couple of times when I needed it to work and I no longer use it or rely on it a second backup app or plan.
IMO, the best backup plan is having a hard drive or two OR large capacity USB Flash drive or two and using a data cloning app such as CarbonCopyCloner or SuperDuper, create exact and bootable clones of your entire system on these drive/s. I am a little lazy, I do only bi-weekly cloning of my main system if I have been doing a lot of. work or projects on my system. If I am really busy with projects I will clone my system once a week.
Realistically, you can/should create new clones of your system on a daily basis, but I regularly burn discs of data and projects I want to save, so I feel that a complete cloning of my system every two weeks has been working for me. I have had this system working for me for quite aa few years and hasn't ever been an issue.
You can decide how often you want to clone your own data and system. Just make it a regular habit if you spend enough time working in front of your computer.
Sorry I couldn't be of more help to you.