If they don't move out on time they forfeit their remaining proceeds.

Either bail and possibly forfeit any deposit you may have given, or do some more research to determine the true value of the house (comps may be accurately reflect that value of you house or may be low due to condition or current market) and if you are confident that you bought it at the right price for you, then proceed.

I am well past the 10-day option period to back out of the deal without penalty, so I assume I would forfeit my escrow deposit if I walked away now.

AND If I let her move out early - do I make any concessions to the move out clause > 60 days notice + one month rent + 50% of deposit forfeited? 

We offered $25,000 over asking and forfeited my commission.

We left our offer at the same price, and could have forfeited this huge deal by only $5k.

Maybe you can incentivize the seller/listing agent to move forward by forfeiting your commission if possible.

If you pay more than an independent appraiser values the property then you would be paying above retail.  2) Seeing that a property investor should not pay above retail, the ethical exit is to forfeit your earnest deposit.  

Because if you were happy with the house prior to discovering the lot was smaller than stated then just buy it...or forfeit your deposit if you think it will eat at you forever.

If so, consider allowing the appraisal inspection with an addendum that also says earnest money is non refundable and will be forfeited if they do not close on X day (existing closing date).