If I'm reading this correctly, it'd be a 90 min minimum connect time:
[KVS Availability Tool 7.1.9 - Reference: Minimum Connection Time [MCT]: FRA/LH-US]
Code:
DD DI ID II
STANDARD: FRA 045 045 045 045
EXCEPTIONS:
A/L FLT-NBR EQP DEPARR AREA
LH TRN --- 090 --- ---
US 0001-2199
Not sure on the interline with different PNRs. I don't think they'd have to. Is your FRA-CLT refundable where you wouldn't get stuck with a big change fee or refare if you missed it?