Originally Posted by David-A

1) No problem at all with self printed boarding passess. I've used mobile phone boarding passess as well without problem.

2) If LH are willing to through check the hold baggage, then you should have no problem getting it stamped. I'd bring a print out of your booking, etc.

Note: If you will be carying the items in hand baggage, then it should be stamped at your personal point of departure in FRA.

Note 2: I'm not sure how German authorities handle goods that are a mix of hold and hand baggage and on the same form. As a result, if they will be exported as a mix of hold and hand luggage you might want to get them on separate refund forms. Although obvioulsy this dublicates the refund fees.
[Unless anyone knows how Germany handles refunds where items will be split hold/hand luggage and connecting airports.]