if they were available with one of those larger seats
They only sell them with the one type of saddle, Walt. It really is a mountain fat bike. But, of course, you can always replace the saddle. They offer a spring-loaded seat post too as an option.
Is there a key operated dead switch?
You can easily turn off the battery, which is key-locked into the frame. Even without that, the bike is programmable for a certain amount of minutes to turn itself off. Unless you know how to use the controller, you wouldn't even be able to turn it on (it's not obvious). And, finally, I now have an Apple Airtag embedded in the battery so that if someone walks away with it, I'll be able to track it.
All of that is a complicated answer to a simple question... If someone wants the bike, they'll be able to take it. If I were in an urban area, I'd probably have a chain lock or something... Also not fool-proof, but security is not a big issue for me where I live.
I may put some more thought into that question once I have a trailer hitch installed on my car so I can take the bike with me. Right now, I start/end at home.