1. Why have you chosen Linux over other operating systems?
There is no general need to chose Linux over other operating systems. All systems have their own pros and cons and you should know them and do your choice based on what's best for your use case.
2. Does Linux have any mentionable downsides? If so, what?
If it comes to fully blown operating systems there is no such thing like "the Linux". "Linux" describes the kernel and the basic philosophy. There are many distibutions out there which have big differences to each other, putting focus on different areas. This can be an advantage since you have the choice, but it's as well the reason that Linux isn't standardized and same things may possibly work complete different on another distribution.
3. Is it possible for a total beginner to start using Linux without any major difficulties?
I've given CentOS machines to 8/9year old kids who were begging for a computer. It was a decision against Windows to force them to understand computer technology instead of just sticking to youtube/co.
4. How has Linux developed during the years?
Linux is in a evolution process, it's moving on.
Over the years Linux became more and more competitive compared to other systems. In some areas it's better than the competition, in others it's lacking.
5. Which distro is your favorite?
Depending on the use case, for most projects I'm working on I use RHEL/CentOS and Debian for servers, openSUSE and ubuntu for desktops, Knoppix for maintenance and rescue tools.