SK AAR

It was definitely SQ that had the obligation to pay EC 261/04 comp. The OP had confirmed F seats on SQ26, but SQ had to cancel/delay this flight and rebook to LH with only 1 F seat available and the OP got a J seat, i.e. SQ downgraded the OP.

The downgrade is not an LH or UA issue. Yes, LH was liable for the initial cancellation of the LH flight ex PRG but apparently this was caused by adverse weather exempting LH from paying EC 261/04 comp. However, LH was still under a duty of care, i.e. to arrange for hotel and meals at FRA, i.e. the OP should have sought reimbursement of the these costs from LH. Once rebooked with SQ in F on SQ26, the subsequent events was no longer the responsibility of LH.

I assume that SQ somehow compensated UA for the miles that UA returned to the OP. Despite being the ticketing carrier UA has nothing to do with issues that occured with the operating carriers.

The OP should be very happy with this outcome. If SQ wanted to play this by the book, the OP could have been rebooked to SQ26 in F the following day (or any other flight with F when ever such seat was available) to avoid the downgrade issue, i.e. exposing SQ only to comp. for delay - but apparently this was never considered/an option to SQ - or SQ could have argued that the OP voluntarily accepted the downgrade to J for the LH EWR flight to minimise the delay/to avoid yet another night at FRA, i.e. no downgrade issue.