I heard that person actively contributed for something like 2 years, providing actually useful contributions, to gain the level of trust needed to plant that backdoor. Feels a bit too much to chalk it up to boredom.
As for the second part, that’s an interesting question. Are there lots of backdoors and we just happened to notice this one, or are backdoors very rare exactly because we’d have found them out soon like in this case?
Yes, you can run Linux in a VM.
But also: you should be able to access your Windows partition from Linux, as it supports NTFS and FAT filesystems, and view the files there.
What I do is I have one partition with Windows, one with Linux, and a third one (with an NTFS file system) for the files I need to access from both.