Keywardens actually also have a chance to drop a different key. I got the Key of Bones off the act 2 warden twice in a row and thought for sure it was bugged, till I looked it up and saw they had a small chance to drop a different key. Sure enough, I ran him again and got his normal key (Gluttony).
Also, if you learn the plans, I believe you should never have them drop again. (Both keys and plans are completely separate from the ones at level 60, if that is the source of the confusion).
Your calculations are flawed. You are counting the bonus experience/gold find/magic find as though that is all there is.
Assuming a ruby in your helm, 5 NV stacks, and a decent paragon level/MF gear, your starting stats probably look like
200% experience,
250% MF/GF
so in your chart instead of xp going 10, 20, 30, 45, it should go 210, 220, 230, 245, same for MF/GF.
If you make that change it becomes pretty clear that you should just stay in ML 0 for everything.
Even if you assume you are naked with no NV, the base 100% experience gain and magic find you have would make the optimal ML be 0.
However, that is assuming you spend 100% of your time dealing damage to monsters. The reason higher monster levels can be beneficial is because more time spent killing each monster means a lower percentage travel time. Even then, most people are probably best off in ML1 or ML2... the only real reason to go higher is for keys (which really do roughly follow your xp chart, making the best ML 3 or 4) or just because it's fun.
1
Also, if you learn the plans, I believe you should never have them drop again. (Both keys and plans are completely separate from the ones at level 60, if that is the source of the confusion).
1
Assuming a ruby in your helm, 5 NV stacks, and a decent paragon level/MF gear, your starting stats probably look like
200% experience,
250% MF/GF
so in your chart instead of xp going 10, 20, 30, 45, it should go 210, 220, 230, 245, same for MF/GF.
If you make that change it becomes pretty clear that you should just stay in ML 0 for everything.
Even if you assume you are naked with no NV, the base 100% experience gain and magic find you have would make the optimal ML be 0.
However, that is assuming you spend 100% of your time dealing damage to monsters. The reason higher monster levels can be beneficial is because more time spent killing each monster means a lower percentage travel time. Even then, most people are probably best off in ML1 or ML2... the only real reason to go higher is for keys (which really do roughly follow your xp chart, making the best ML 3 or 4) or just because it's fun.