This won't solve the whole mystery, but would help get some real clarity--go to the airport and talk to the ticket counter. We have done this on two cases when travelling overseas and the agents were helpful and insightful. And in both cases proved that their call centers were 'wrong' in that 'everything would be okay'. I wouldn't trust the call centers since they aren't the gatekeepers--the ticketing agents are, so check with them.

The problem is that you're facing this twice--once possibly in FRA where you can't talk to them beforehand. For this, I would keep searching for firsthand information. I know in April my wife flew through FRA on UA ticket, LH metal with a visa and had no issues. OCI I know was even less of a hassle back then, but this is a possibly dated data point today.