Downgrade from Catalina to High Sierra or lower - process

Hello everyone,

I am using a MacBook Air (mid-2015) for about 4 years now and today was my first time updating the OS version. I must admit that, however excited I was to discover the new Catalina OS, I soon regretted clicking on "update". Indeed, as far as my MacBook is concerned, the main problems are that the dictionary app doesn't work anymore and, most importantly, that the CPU experienced unprecedented heating (around 70°C to 80°C), whereas is hardly reached 69°C when running El Capitan.

For those reasons, I wanted to downgrade to a lower version, but it seems that, as I don't use Time Machine, this process might be giving me a hard time. I abandoned Windows computers several years ago and I not used (nor willing) to go through a whole installation process (reinstalling Apps, customize everything, ...).

I have two precise questions for expert users around here:

1. Since Catalina divided my HD into two partitions (Macintosch HD and Macintosch HD - Data), would it be possible to erase the Macintosch HD partition and reinstall a previous OS version with all Apps and settings safely preserved on the second partition, so that I would just reopened them as if nothing had happened?
2. If I create a Time Machine backup now, on Catalina, will I be able to restore my environment along with Apps and their parameters into a lower version. I expect it to be the best option, but I fear it might not restore all my Apps and data.

I hope that this question is clear enough.

Thanks a lot to whoever took the time to read this topic.