you made the right choice with multiple batteries if you're doing short SIT's. I probably would have bit the extra money and got the 4-gang charger so they all charge together, since series charging can get really annoying if you get busy and it also lets them sit on the charger longer to balance all the way out, but there is no way you were going to do what you wanted to do with recharging mid-day.
Running in the cold affects the burn time of the batteries, but they will still throw off a good bit of heat and I think you'll be surprised how not-cold they are when you pop them out
You're right on the issue of the gang battery charger; I just cancelled the single charger order & will have to see if the Amazon dealer will accept the cancellation. Then I'll order up the gang-charger.
On a side note - how do you tech folks mount a dive light on these scooters? The deeper rivers are pretty dark, with lots of sunken logs for obstructions, so a good light is definitely necessary.