Was the truck unlocked? Otherwise hard for someone to get under the hood to mess with the battery connections. From what you've been told it sounds like the vandal was someone who really has no idea how cars work and was just going off what they thought would work.
Chances are the sugar in the tank won't do much of anything. Still, you may need to change your fuel filter at some point. Fortunately those are cheap. I doubt the vandal put bleach in the tank as it is not as well known of a sabotage, but if they did it will rust everything it contacts badly. The first gen Explorer has a plastic gas tank which helps, I'm not sure on the second gen but if it's still plastic then you mainly would need to worry about anything that gas was in contact with such as the fuel pump assembly.
I would drain the tank of fuel and put all fresh fuel in just to be safe.
On to things they actually could have done... check your oil level, hopefully they didn't drain your oil. Also check the transfer case if you have a 4WD model. If the fluid is drained from either of these it will be bad news if you drive your car before refilling them.
As for the electrical aspect, check under the hood. If the truck was locked the vandal may not have actually touched anything there. If not, look for anything out of the ordinary such as cut spark plug wires, connections unplugged, and so on. Check that the battery connections are tight; most battery connections require tools to remove so a vandal won't waste the time. Also check that the battery connections are red wire to positive terminal (+) and black wire to negative terminal (-).
I kind of get the vibe that whoever messed with your vehicle isn't exactly all there in the head, and a lot of what they are saying they did is just talk. Hopefully nothing has been majorly tampered with. The stuff you mentioned about headlights cutting out and the computer frying, is all baloney - it would happen immediately, if at all. The person who told you those things wants you to lose faith in the vehicle, and make you scared to drive it for fear of an unexpected failure.
Do NOT let this person have a second chance. If they have a mental disorder, maybe the cops can get them the help they need. If it is a violence thing, like drugs or alcohol induced, they are a bad person to ever be around and the cops may not be able to help. Put as much distance between you and them as you can, and keep it that way. Investing in a firearm as protection is a good idea, and while I'm not suggesting using it on someone, let them know that you have it and know how to use it.
A person's vehicle is as much their personal space as their home. If they invade your vehicle they might as well be invading your home, the way I look at it. This is nothing to mess around with.
