1. I have been to RA 10 times, flown Jakarta to Sorong about 8 times, twice from Denpasar, Bali to Sorong (but that is not ideal route as it requires a stop over, either in Makasar or Manado). Jakarta to Sorong flights had always been on time (knock on wood) because the red-eye flights. Flight delays usually happen later on in the days.
I hope you picked Garuda flight as their gates are in Terminal 3, the same terminal for international flights. If you go on Batik, its gate is in Terminal 2, which requires a ground transfer. Sky train is available for that, but it stops operation at around 9 PM. You may resort to a taxi or bus if you go to terminal 2 after 9PM.
If you want to go a day early to Sorong, there are a couple recommended hotels in Sorong; 1) Swiss Belhotel Sorong (where I usually stay, 2) Aston Sorong near the airport.
2. As far as check-in luggage, if you fly with Garuda, they will allow you to bring 2 bags (20kg max each) free with the 2nd bag is for your dive gear. You can tell them at the check-in counter which bag is you dive-gear bag. For me, I travel light, only 1 check-in bag, 2 carryon bags (a backpack & a small carryon bag for my reg, rash guards & a set of clothes just in case my check-in bag is lost or got delayed during transit. This happened twice in 19 years of traveling overseas),
3. I usually get cold after 6 days of divings and add a 2mm shorty over my 3mm wetsuit for the 2nd 6 days of diving. Don’t dive with only shorty as RA hydroids are abundant, itch like hell and will blister up your exposed ankle’s skin when your leg accidentally rubbing against them. If that happens to you, pour some vinegar when you get back to the mother boat.