Setting up a practice panel: who do you invite, and how long do you keep it?

I'm planning my first live practice interview with real people before placement interviews start, and I'm stuck on the basics.

Who to invite? I have a senior who works as a backend developer, a friend who's brutally honest, and a cousin in HR. Is three people too many for one call, or is that actually closer to a real panel? I'm worried I'll spend the whole time answering one person.

How long? 15 minutes feels too short to get past "tell me about yourself", but an hour feels like a lot to ask of people doing me a favour. Is 30 minutes the sweet spot for a first round?

Briefing them. Do you send your panel your résumé and a list of areas to push on, or let them ask whatever they want? I've read that it's better if they stay in character and save feedback for the end.

If you've run a practice panel, what worked, and what would you do differently? There's also a guide on running a mock interview with friends if anyone else is setting one up.