This is not lufthansa's fault at all.

first of all, when buying non changeable tickets, one should be aware that they are non changeable.

when buying 2 tickets, they are independent and a delay on ticket 1 does not cause any obligation to the airline of ticket 2 at all.

when buying from a travel agent, he has to deal with any changes (unless in case of IROPS, which is not the case here).

so you bought the tickets from a crappy OTA as it appears - not lufthansa's fault at all but yours for reasons 1+2 and the OTA's for reason 3.