2 separate tickets and valid double-entry business visa - Input Please

I have two separate air tickets
1) PEK-BKK-PEK arriving in China from Thailand on April 21st at 00:20
2) PEK-NRT-PEK departing China on April 21st at 16:00 (around 16 hours later)

Currently I also have a double entry business M type which has been stamped once (i.e. one entry used).

When I arrive back in PEK from BKK and just before I need to go to NRT can I ask immigration to put me on a 72-hour free stamp and NOT stamp my double entry business M type? If so do I need to prepare any documents other then my flight ticket?

I need to use the second stamp of this business M type when I return to PEK from NRT and hope they will not use it between my BKK and NRT trips. Otherwise I have a big issue.

Do I interpret correctly that I should be okay to receive a 72-hour free transit between these two tickets?