Level 3 charging and using Cabin heater

Recently I was sitting late at night at the Nissan dealer charging my 2023 SV plus on their level 3 charger. I needed the charge to return home almost 100 miles away. It took more than an 1.5 hrs to reach a comfortable level of charge on the battery before I left for home.
The Temperature was 5 degrees F outside, so my dilemma was: Would running the heater make me sit and wait even longer to recharge the battery, or would the Level 3 charger and/or the car manage the electrical charging and keep charging the batteries at an optimal rate and take roughly the same time to reach the same level of charge? I wanted to get home soon, so i sat in a very cold car.

Postulating here, but the heat pump heater in your car probably draws only 2 to 2.5 kW at max output (my + has the resistance heater so no actual data to quote), so IF the DCFC was charging your car at around 30-40 kW the heater would have been using a small fraction of the total. My guess is that the DCFC Level 3 system was slowing way down because of the cold, although my 2019 has accepted at least 30 kW to charge to around 80% SOC before slowing down at cold temps. 80% should have been plenty to get home? Anyway, the cold is never your friend in a Leaf but this seems worse than my experience.

Would running the heater make me sit and wait even longer to recharge the battery, or would the Level 3 charger and/or the car manage the electrical charging and keep charging the batteries at an optimal rate and take roughly the same time to reach the same level of charge?

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That's a good question and IMO it all comes down to how the controller is programmed. It could work either way (or somewhere in the middle) and the only way to know would be to test it.

I am sure there is a yes or no answer to my question, but my guess is that it would be close to impossible to find the one Nissan engineer that actually knows the answer to the design of the charging management software. I haven't tried the dealership because i seriously doubt that neither the service nor sales would have a clue.

The LBC manages the charging current to keep cell voltage below the maximum allowable level. Therefore, the net current into the battery should be the same whether the HVAC (and 12V accessories) are in use or not so you might as well be comfortable during future charging sessions. As noted by dmacarthur, the energy use of the HVAC system is small compared to the DCQC charging energy.