If your luggage is tagged all the way through to HKT, you do not need to clear customs at BKK. You do however need to clear immigration (passport control) at BKK. There are two ways to do this. The first way is to go through the normal immigration channels which lead to baggage claim and then to customs and exit to the landside area. The second way is if you have an onward boarding pass to CNX, CEI, HKT, KBV, HDY, or USM is to go through the special international-to-domestic checkpoint, located at the entrance to Concourse B, Level 2. If doing that, you will first have your boarding pass checked by the airline staff, then go through passport control, and onto your domestic gate. At your final destination, you collect your bags from the international baggage claim, and exit customs there.

If your luggage is tagged all the way through to HKT, but you exit customs at BKK, then you obviously will not have been able to collect your bags at BKK and will still need to collect them at HKT from the international baggage claim. At least I presume that is what will happen, I've never actually tried it. I doubt TG would let you ask your luggage to be rerouted to the domestic baggage claim being it hasn't been passed through customs.

If your luggage is tagged only to BKK, then you must pass through the regular immigration channels, to baggage claim, and through customs at BKK. There is no way I know of to recheck your baggage until about 0500 when the domestic check-in counters officially open. Normally though I've seen staff arrive about 0430 - 0445 and open up a counter or two at that time and start processing passengers. There likely though will already be quite a few passengers waiting, but nobody knows which counters will open up first. As soon as one employee comes, everyone rushes towards that counter, and if your unlucky and are at the end of the line, you'll likely still be in line when the full number of counters open at about 0500. If you're staying at the Novotel though, there's no reason not to simply bring your luggage with you, have them load it in the shuttle van, have a bell boy help you with it at the Novotel, then reload it in the van in the morning.

If it was me though, and I didn't need my luggage that night, I'd ask when checking in for the flight to BKK if it can be sent to HKT even though exiting at BKK to go to the hotel. My guess is they'll allow it.

As for the tunnel to the airport train and Novotel, it's not open yet. I was at BKK on Sunday and passed by the travelator down to that level and it was still closed off. I imagine once the regular train service starts, the tunnel will be open to public access.