Question about trial accounts

Howdy. I recently got a friend to agree to try playing WoW with me. They went through the official website, clicked the “try for free” button, and set up a trial account. Unfortunately they’re not really in a position to pay a ton of money for the game just yet, which means for now they’re stuck on the trial account. It’s been a long time since I myself have had to deal with trial accounts, so I was hoping if anyone has had some more recent experiences with them I might be able to get some answers to a few questions.

  1. As an old-time player with a regular account (haven’t pre-purchased DF yet though if that makes any difference), will I be able to play with my friend while they’re on their trial account or will our characters appear out of phase with each other?
  2. Will I be able to send mail or trade with my friend as long as they’re on the trial account?
  3. If and when my friend decides to upgrade from a trial account, how exactly will that work? Will they have to purchase the base game but also purchase DF/Shadowlands (whichever’s current at the time) separately from that, or do they just have to buy the current expansion? Or, alternatively, do they just set up their monthly subscription and, boom, their account is regularized?
  4. If I can send them mail/trade on their trial account and if all they have to do to upgrade from a trial account is subscribe, would their account upgrade if I sent them gold for a WoW token and they bought it? (Now that I’m thinking about it I think trial accounts used to be unable to use the auction house; dunno if that’s still the case though)
  5. Last, easy question, just to make sure I’ve got it right: the max level for trial account characters is still twenty, correct?

If there’s a specific section where I can read more about how trial accounts work somewhere on the official website, please let me know; I’d love to go through it myself. Thanks in advance for any and all help!

Play for free up to level 20.
If you don’t turn off experance you will get up to level 30 when/if you put time on account.
You can talk in groups and battle.net friends; chat.
Make and spend up to 1000 gold.

Basically play alot of content for free.
Try all the classes have fun.

The restrictions are few.
With one month subscription you can do the achevement to use Cromie Time.
And with just a sub you can play up to 50.
If you want to continue with a free play style. I recommend you lock experience before level 20.