Does a player's workload have any effect on retirement probability. Like say is a HB that is getting 300-400 carries a season more likely to retire early than a HB that is getting more like 150-200?

To piggy back off you.... I'd like to know if the same can be said for injuries!

Injuries yes - if they are severe. Work load, no (although a heavy work load could potentially increase severe injuries)

At what point of the sim do we know if a player is retiring and do we get a notice of this? At seasons end, just before draft, after draft?

After the End of Season sim.