My tickets say "Not valid for entry via mobile device."

No need to worry! Some venues will print this on the ticket to discourage people from zooming in and duplicating the barcodes on the PDF that is on their phone! This doesn't mean that the venue will not allow the use of the mobile entry barcode through the SeatGeek app. If the barcode is generated and displays in the SeatGeek app, you'll be good to go!