arriving 6:30am sun, leaving 11hrs later - 5:30pm? That's tough. You'll have to be heading for the airport by 4pm, which means starting your last session 3pm or even 2:30 if you're not right by the airport express, so it's tight.
Best bet is to pick some that are 24hr. You'd have to wake them, but at least they're there.
Dunno about HGs - you could call a day before and ask if they're free (say) 10am or 11am the next day - just make sure it's clear you're both talking am not pm. And have at least one contingency plan.
Or alternatively you could call a 141 girl directly and make a booking the day before. Someone like Shirley or Niki in Sheung Wan (not far from the Airport Express in Central) would be able to do that and is likely to keep a commitment.