Hi Joel, this is a good question. Regarding your first point, I can't find any SOPs or other guidance specifically around stacking; however there are some related resources that you might be interested in.
EVM Standard operating procedure (SOP) E6-06.1 “Storing vaccines and water packs in refrigerators and freezers”
https://www.who.int/teams/immunization-vaccines-and-biologicals/essential-programme-on-immunization/supply-chain/effective-vaccine-management-
There are some interesting diagrams, such as this one for storing vaccine in a front-opening refrigerator.
Two other resources may also be relevant.
Training for mid-level managers (MLM)
Module 1. Cold chain, vaccines and safe-injection equipment management
"Make sure that vaccines, diluents and ice packs are loaded according to existing national guidelines."
"Application of EEFO".
https://www.who.int/immunization/documents/MLM_module1.pdf
Immunization Academy video
Arranging Vaccines Inside Any Refrigerator
2 cms is recommended as the minimum distance between stacks.
https://watch.immunizationacademy.com/en/videos/119
Regarding your second question around whether the temperature distribution in the refrigerator can be affected by stacking vaccines, and therefore affect vaccine potency, this is a somewhat tricky issue. In principle, stacking in the fridge shouldn’t be a problem thermodynamically because the volume of a fridge is too small to permit the creation of micro-climates within the vaccine storage compartment.
However, if the stacking is such that the thermostat sensor gets covered/blocked by secondary packaging, then there might be an issue with the fridge’s temperature control, mostly caused by a compressor overdrive, leading to freezing. This will of course depend on the make/mode and placement of the thermostat sensor and is a very unlikely scenario. If there is any doubt, a LogTag or similar logger could be placed in a warm part of the refrigerator near the top or a cold part near the bottom and the maximum/minimum values checked on a dailiy basis.
Over-stacking may be more of a vaccine management problem. For example, can a health worker ensure the EEFO policy in a fully loaded fridge?