grand-theft-auto-online
The requirements for playing the heists are plastered all over the internet:
However, some places says you need to be level 12 to host the heist where as other places says that heists unlock at level 12.
What is correct?
A level 6 (example), can he join a heist, provided that someone else that meets the requirements hosts it? Or does everybody need to be level 12 (or higher) and have a high-end apartment?
it is definitely known. you need a high end apartment with a planning room. $200k and up should be fine as all the high end apartments are identical inside. once heists are enabled you will be good to go.
A similar thing happened to me, but in a worse condition - all my progress was gone; my rank was reset to 1, and i lost my cars, and apartment. I recommend you to contact rockstar support, by posting a question at https://support.rockstargames.com/hc/en-us . They restored my progress within 5 hours I posted the question.... Here's the mail i received- 
Best Answer
You can join someone else's heist if they invite you, regardless of level or residence.
you only NEED to be level 12 and own an apartment to be the heist LEADER / HOST the heists.