The problem with your calculation is that most cargo is bulky especially what you’re taking about, medical supplies, ventilators etc, you won’t get 16 T of cargo carrying it on seats. SAS has a history of carrying I think mail bags in seat pouches I think but that’s about it I believe. Another problem is that you’ve got no main deck loading door for Hi loader so it would take a very long time to on/ off load cargo, and no fire extinguisher system or main deck smoke detection system would be available and how would you restrain the cargo on the seats, that’s why cargo conversions costs millions of pounds.
As an experienced freightdog, a fully kitted out 737 freighter with all the kit inc the above, side cargo door, and strengthened floor, and loading the cargo in main deck containers, you sometimes bulk out at 17-20 tons, carrying 16 T in a pax 737 would definitely not work unless you were carrying lead in the seat pockets. Channex 737s are the QC conversions, so can remove seats and carry cargo and are fitted with the side cargo door and are certified for this use.