How does the sim determine when a pinch runner is used? How does it decide who to use?

Anyone have any insight?

I do not know how the SIM determines whether to use a pinch runner, except for the obvious point that your managerial setting will influence frequency.

As to the 2nd question, I would assume that the SIM will choose the player on your bench with the highest speed rating.  I do not think that SB frequency/ability factors into the choice at all.  But I do not know this for certain.

It would be interesting if the sim would use some of those ultra-high-speed Pitchers as Pinch Runners.  But I highly doubt the game has that in its logic tree.  Although that idea is not without historical precedent.  Even today, managers like TLR use speedy pitchers as pinch-runners.