I think the insulation will help retain heat longer from your warm vehicle, but you're always going to be fighting the cold cement slab.
I have blown in cellulose insulation on top of bat insulation in my house to bring it over R-38. It's very easy to do, but it will take two people. I would also suggest putting in some depth reference points in a few places so you can tell when you have enough as it's really easy to go overboard. I didn't do that and ended up putting in almost 50% more than I had planned.
I think your foam sealing is a good idea if you can identify some air gaps. If you have vinyl siding, it's very easy to pop it off with a "zip tool" which should make the job much easier.
The radiant heat blanket idea is interesting. Would you use something like
this?
As far as the tar goes on your walls, it wouldn't be a bad idea, but I'm not sure it's needed. If you do decide to put tar on the walls, it goes on very easily with a paint roller and is rather cheap. Putting dirt around the foundation sounds like a decent idea as long as it doesn't cause water to start splashing on your walls. It might even help with drainage. A few years ago I dug all around the perimeter of my house to the footings, put in a drain tile system, skim coated the walls, put tar over that, and then styrofoam insulation. As I said before, that made a big difference in the temperature in my crawlspace, not to mention keeps it much dryer.