Re: waterproof electrical connections
I'm not sure if you've still looking at options, but one idea is to follow the instructions Jack R. has on EVTV.ME for encasing A123 batteries in plastic using a mold (I can't recall the video, but it shouldn't be difficult to find). I know the current issues with considering A123, but it's possible that the method Jack is using to encase multiple pouch cells could be also used to waterproof the areas you just need waterproofed.
There are other possibilities for waterproofing, but most would cause issues with venting (if necessary) or would be greatly affected by the heat. If A123 ever gets off the ground again, the pouch cells combined with Jack's method could be the best option for waterproofing. The cables could even be attached to the battery before encasing them.