Here's some comments from my experience. The SSN issues doesn't necessarily mean it is a EV related (8yr 100k) problem. I had an SSN issue a year and a half ago where my battery was replaced. However, the week before that diagnosis was made, the dealer diagnosed a relay that was not covered under the warranty. Cost me $100 for the $25 part and $75 for the labor. However, I don't believe for a minute that this was the real issue...but I think that was the best diagnosis they were able to make. This was before I got my OBDII Bluetooth module and Forscan running. No hard feelings...I give the dealer the benefit of the doubt. However, it does reinforce something that I have come to know...there are likely 100s of possible issues that lead to the same symptom...SSN. This is why I purchased the Forscan setup; so I don't have to rely on the dealer to tell me what's wrong.
In this case, I'd try a different dealer. When I had my issues, they didn't charge me to diagnose. Their position was that they would do the diagnosis and then seek payment from me or from Ford depending on the outcome. Sure, you have to trust they do the right thing, but I've been doing pretty good by my dealer...even with the $100 I paid for what I thought was an incorrect diagnosis. But, I think my dealer is much better than the one who wants to be paid for the diagnosis. Could be they don't know how to effectively diagnose the car and want to be paid for their time in case they are not efficient. If possible, find a dealer others have used for EV troubles. I do agree that Forescan is a good first step; but it isn't always a slam dunk.
Here are my Forscan experiences.
1) After I had the battery swap, the car was getting charging faults. Took me 2 days to notice that the temp of the on-board charger was getting too hot...using Forscan. Oringinally I was monitoring the charging current and noticing that it was starting out with a high current but eventually throttling back...eventually down to 1 to 2 AMPs before stopping and issuing a charge fault. I also noticed a funny sound which turned out to be one of the cooling pumps running dry. Eventually I noticed the high temp of the charger using Forescan and then noticed the coolant was very low. Dealer forgot to put coolant back in after swapping the battery. The charge circuit is in the same coolant loop. No cost to me; dealer fixed.
2) After about 1 year after the battery swap, I got SSN again. Got it quite a few times. Using Forscan I could see a communication fault between the TCU and the battery. There was another issue logged, but I don't remember what it was. After taking it to the dealer, they double checked the TSBs and sent me on my way. I got an SSN 2 times on the way home but I was able to get it home. Called the dealer and dropped it off the next day. Got a call mid day that it was ready. They went from connector to connector looking for an issue and found that the main battery connector at the rear passengerside wheel was not fully seated....likely another issue from the battery swap a year earlier. Forscan wasn't too helpful to me at the time, but in retrospect it made sense. No cost to me.
3) I got a yellow wrench thrown. I had this issue off and on well after the 3 year 30k warranty with respect to the cruise control. Cruise control would work, but if I tried to press the "resume" after disengaging with the brake or "cancel", I'd get a yellow wrench. I read the codes with Forscan and it indicated an issue with the steering wheel module. After a wrench was thrown a subsequent time, I noticed that the cruise control wasn't working at all. Later, with no yellow wrench, I noticed that the cruise control wasn't working at all. The error code, when searched online, indicated some issue with other steering wheel controls. Long story short, I suspected that the switch contacts were likely not conducting very well due to lack of use. I ended up just working the switches a bit aggressively and the problem went away. No more yellow wrenches and the Forscan errors are no longer generated....and cruise control is working. There is a good chance it is a coincidence, but at least I do see correlation with Forscan and its ability to help diagnose.
Anyway, for this case, find another dealer :)