I live in Bangkok and someone I'd like to surprise has a 6-hour layover in Changi. I would like to know if the following is possible:

1. I check in online (no bags), clear Immigration and go airside.
2. Surprise said person at their arrival gate.
3. Leave airside together to visit The Jewel or maybe even head into town for a bit (for illustration purposes, leave the practicalities aside)
   1. He would be a transfer passenger
   2. I would be a departing passenger
4. Clear Immigration again to go airside a second time
5. See him off on his flight
6. Proceed to my own flight.

The plan without step 3 was confirmed by Changi and airline staff to be feasible. I also see no reason why a transfer person wouldn't be allowed to leave airside to visit The Jewel. They've invested plenty after all!

But I am wondering how, as a departing passenger, they will allow me to leave airside and come back several hours later yet again?

Thanks for the comments!