Well it's a personal thing - i like to set laps based on how i feel. Could be after 5k or 10k in a 70.3 race.
So manual laps would be best.
So I tested hotkeys for lapping in a multi-sports activity and there's good news and bad news.
Good news: they work
Bad News: 'Lap' actually is 'Back' and goes to the next activity :-(
Not sure if this is a bug or not, I'm going to log it and see if Garmin have anything to say about it.
CW
How did you get it to work
Please read earlier posts, and you will have your answer.