Would the primary/main owner of the Private Key be able to figure out what has happened to his/her money .

Yes, the wallet would show a transaction that spent/moved the coins

find the real name of the person who stole it?

Most likely not, unless using some kind of tracing service (Chainalysis, CipherTrace, Coinfirm)

Can the primary owner open a legal action against this?

Assuming you find out who did it and you can prove it, and your jurisdiction supports such legal actions, then yes.

What's the best course of action for the primary owner to do if this happened to get their money back to find the person who stole their money?

Ask the internet for help, do some analysis yourself (using public facing tools), hire a company specializing in this type of work.