Yes I do know the FMVSS regulation quite well. Knowing the libability that modifications such as this have can help with deciding if they are really worth risking a legal action that cannot be won...
If you do decide to sell this, make sure you havesuficient liability insurance for the possibility that someone is hit and killed because they did not hear the car approaching.
The issue is that the replacement parts are no longer available (someone at Nissan underestimated how many to make at the final production run)
Sure there were other 30kWh cars that have had a replacement pack, but that was then, and now the parts are no longer available. This is why Nissan is...
Unfortunately there just isn't a business case for Nissan to offer upgrades for the older "obsolete" LEAFs. This is an age old issue with all sorts of electronic or leading-edge products with support after the warranty periods, especially if the product is a low volume one as the LEAF is. This...
The biggest issue for the 1st Generation LEAF batteries is that Nissan stopped making them a long time ago. There simply aren't any to be had. This is why Nissan is offering buyouts on these cars. Take the buyout and buy yourself a newer BEV.
Here is an article that talks about the BMS from a few years ago. Scroll down to the part of the article "Under the hood and under the floor":
https://insideevs.com/reviews/354688/2019-nissan-leaf-40-kwh-test-drive/
One way I found to keep the cells all balanced with each other is to periodically charge the car to 100%, then charge it again, and then again. 3-4 charges after it is at "100%" . This will cause all of the lower charge cells to balance up with the other cells.
The 24 kWh battery has been obsolete for several years now, and Nissan made a final production run of the battery and its components in 2019. I would advise to take the offer, counter if your haggling skills are good, but by all means take the offer.
You will also get your local utilities approval to use the V2H connections. The limited use of V2H in the US has been limited to commercial trials, and the interface hardware necessary on your home side is limited, very expensive, and needs approval from your utility.
+1 on sagebrush's question. New type of cells and active thermal management, combined with local garage? sounds like something that should be examined very closely before delivering a verdict.