Visa Free from HK to Singapore via Chengdu?

US citizen here, itinerary as follows:

Depart Hong Kong 11 PM on Friday, July 13 > Chengdu (CTU) | Hong Kong Airlines
Depart Chengdu (CTU) 11:55 PM on Monday, July 16 > Singapore Changi via Kuala Lumpur | Air Asia

According to the rules on this page, I should be fine to receive a 72-hour free transit when I arrive.

I'd like to validate that: (1) Origin in HK Is okay?; (2) On a separate ticket, I have traveled from SG to HK, making this a round trip, but on two different tickets. This should be allowed, correct?; (3) 23:55 departure is cutting it close to the midnight deadline. Will this be a problem? As long as I make it through immigration and my flight is not canceled, it shouldn't matter if minor delays keep me in country past 00:00?; (4) Technically staying for 73 hours, but the clock starts at midnight the day after landing in Chengdu, right?; (5) I have booked through a travel agency - this should not cause any issues?; (6) Not a problem that my flights in and out of Chegdu are with separate airlines?

I have tried calling the airlines, travel agency, and the Chengdu airport to assist with these questions, but have not gotten much help from any of them. Please advise.

Thanks!