Firstly all the pulleys can only line up at one height setting, generally one in the middle.
Secondly one of the reasons for using a V belt is so they can be out of alignment.
The tensioning arm can usually swing a little to allow this to happen with out the belt jumping off.
Thirdly a strait snap comes from a sudden shock loading greater than the belt can stand.
It is more likely to happen if the belt is too tight and one of the spindles is jambed.
A bad belt angle on ground engaging decks is generally because the deck wheels are loose, mal adjusted or just plain worn out
They get very sloppy around the axels but no one replaces them till they are so worn that they no longer roll and leave drag marks in the grass.
You have it right, worn ground wheels usually make the deck sit lower.
The idler pulley should have a spacer inside the bearing .
There are hundreds of different idlers where the only difference is the height of this spacer.
So was the old one the same as the new one ? It is not uncommon to find the wrong pulley in the right box.
Malaligned pulleys may cause the belt to roll only if it is too tight as can the wrong idler.
Some Idlers are very expensive while a very similar idler is 1/2 the price that may or may not work properly.
Briggs & Stratton owns Simplicity so nothing suspicious of getting a part in a Briggs Bag.
Photos on the web are oft decades old.
Chiming in on the idler pulley that looks to be wrong. It is a v-belt idler when it should be a flat idler pulley. I could be wrong but the back of the belt is in a v-pulley, not a flat pulley. Picture attached of the idler that I think is wrong and the type of idler I think it should be.
Wrong pulley
View attachment 40485
Right pulley. It does not have to be plastic but that was the best picture I could find for a flat idler pulley.
View attachment 40486