If forfeitures are reallocated in a Profit Sharing Plan, can a portion be used to pay administrative expenses (assuming the document allows this as an option)?

Yes, but only if the expenses are authorized under ERISA. The DOL has issued several opinion letters on when you can use plan assets to pay plan expenses. See, e.g., DOL opinion letter 2001-01. I think you can find it on the DOL website.

I believe that administrative expenses (i.e.-payments to a TPA for compliance, document, or reporting work) are not considered to be a settlor expense. Since paying for a TPA to monitor and help the plan maintain its tax qualified status is necessary for the ongoing operation of the plan, as long as the expenses are not unreasonable, I don't see where you would have a problem offsetting with forfeitures.

The decision as to whether to use plan assets to pay plan expenses is a fiduciary determination. So only fiduciaries of a plan should be making that call for such plan.