Conclusion to this topic...I have now reached near the end of the regular season and my playoff position is secure no matter what I do the final 3 games. 5 or 6 of my starters have rested for the past 10 games and are now up at 100% (some just getting to 100% recently). So my question is, do they need to play the last few games to get the rust off or does this game not get that complex and they will be hitting normally come playoffs?

Rust does not exist.

Off days help rust though don't they? If I rest a guy the day before an off day will I get credit for two days rest? I would think so but I guess I'm not sure.

A day off is a day of rest yes. Exception: in the post season you'll see a 'placeholder day' on the schedule. That is not a rest day.

Perhaps slightly off topic but what constitutes rest? Does fatigue occur from ABs or Games Played? And if it's games played does a late inning substitution count the same as a full game played?

No one is answering because we're not 100% sure. Innings and AB are my guess. But DH doesn't seem to get much of an extra bonus rest from not playing the field and, IMO, SS/C fatigue at a slightly faster rate.

I would agree. It seems that if guys are in PH/DR duty they still seem to recover some.