Would you know a virus if you saw it?

If the kernel is compromised all bets are off. They have complete control of your machine.

Depends on the malware.

With total access, nothing would prevent the malicious code from modifying the task viewer itself to make it ignore the resources it is using.

Accounting for every way malware might be discovered is difficult, but with enough system access, it's all possible.

https://www.youtube.com/watch?v=c-ftuiRDqO0

in theory, a virus can hide from them if it replaces (or LD_PRELOADs) them with something that filters the virus from its output.

those task viewers will show everything running, except some kernel modules, because they are part of the kernel.

also fast cron jobs will only be visible for a very short time. only while they run. if they run for like 250ms you probably won't notice them.