That's not how MR works. MR reduces damage by a percent. 100 MR makes Magic Damage deal 50% as much, 200 makes it deal 33.333% as much, and 300 makes it deal 25% as much.
Swain's Q does 300 damage. If they have 0 MR, each tick (there are 6, once every 1/2 second) will deal 50 damage, for 300 total damage.
If they have 100 MR, each tick will deal 25 damage, for 150 total damage.
If you use Swain's E to deal 300 damage, all at once, and they have 0 MR, they will take 300 damage immediately. If they have 100 MR, they will take 150 damage immediately. (When I say immediately, I mean after the short delay on the W, they take all the damage at once)
Also, as you don't have preconceptions, so you'll be more willing to listen-
Armor and MR do NOT have diminishing returns. What happens, is that as you get more MR/Armor, HP becomes more valuable. The Armor and MR are always worth the same amount, regardless of how much you already have (unless the enemy is reducing/penetrating through all of it if you have a very small amount, but that's almost never going to happen), whereas HP gets more valuable as you level up. Therefore, assuming you will only be taking one type of damage, Armor/MR is more valuable at first, and HP will eventually become worth more. Due to the cost of the stats, though, getting HP if you are at 2K already, is only worth it if you already have 70-ish in both resistances.
All assumptions regarding buying resistance versus HP assumes you take exactly even Magic and physical damage. You usually take more physical damage, so armor is generally worth more than MR, and HP versus resistance value is usually rather complex, so you have to eyeball it.
Also, True Damage ignores all resistance, so a team with Darius and Irelia makes HP more valuable than usual.
Edit: Also, the effective health formula, which determines what percent of damage the resistance blocks, is 100/(100+X), where X is the relevant resistance.