Will arrive at 7 pm in HKG and fly out at 8 am the next morning, I'm travelling on one ticket not two separate tickets. I plan on staying at the airport and I will meet a school friend who is now living in HKG, briefly but only at the airport. I have tons of questions:-

Should I check my luggage to HKG or to my final destination?

I'm OW Sapphire, how can I visit the lounge, shower, etc and them go through immigration to visit the school friend? After the visit I want to return airside and hit the lounge. What time does immigration close? Will they let me through with a boarding pass for travel the next day? Should I just ask CX staff for assistance once I arrive in HKG?

Where is the best place to stay in the airport overnight once the lounges close?

I've been to HKG and am familiar with the airport but have never transited there.

Any help will be appreciated. Thanks in advance.