From my understanding, Int2Int transfers work similar to that of the US, pass immigration, collect bags, customs (do they still have the Traffic light Red/Green for full customs button?), security, etc... So not that different than an I2I in the USA, also if someone booked the same-day turn in NRT doubt they would have more than a carry-on.
From an airport perspective, both boarding passes should be printed either online or at the airport (Covid checks, etc), so at MEX it's a matter of following signs (Conexión or Transito Internacional,etc), would I schedule a 60 min connection (not sure what MCT is off the top my head) probably not, but 120 would probably be enough and as in most airports if a flight is running behind you can always ask nicely and sometimes move towards the front of the line, etc (with or without MEX airport staff)
The real question is whether Japan will be open in October/November (sidebar: technically this fare is available in Dec/Jan but at 2500+ versus 900), Australia opened today, Singapore is starting to open up, one could argue that if there isn't another massive spike (Omicron v3?) we could see non-China markets (that's a whole different ball game - mainland and HKG) start to be open by late 2022...