The spell seems to be Strength of Earth, the first spell from the Earth school. According to the Wiki it increases Strength by at least 4, which should be enough according to you. When I get my next level up, I will give it a try and then report back if it worked.
You are correct. Furthermore, the spell can be stacked, meaning that you can cast it multiple times upon yourself (up to your spell maintenance limit, which is based upon your IN).
(Still, it feels odd that the game doesn't seem to offer you a possibility to have your follower put the boulder in the wagon.)
Not to spoil, but there is at least one quest in the game which also involves carrying a large and very heavy object, and for which a quest-specific follower has been made available should you need him. So the programming is there, but only under very specific circumstances.
Since I can do all these things, also being able to put an item from their inventory in a nearby container, would make sense. But then, it is so a specific action with so litte purposeful uses in the game, that I can't fault the developers for not implementing it ;-)
My hunch is that it has more to do with how the game AI handles regular (non-quest specific) followers: they're inquisitive and greedy enough to scavenge things off the ground and hold onto them, including tech components (such as plants) and enemy drops which you would otherwise consider junk. (It's probably been designed that way to prevent them from getting rid of anything you actually want them to keep.) So if you do get them to pick up a big heavy stone, they're not inclined to release it to anyone else except you.