If the LH ticket had been correctly updated/reissued, the OP would never had faced problems that occurred at FRA. My bet is that the OP never had valid ticket for the UA47 flight until someone resolved this at FRA - but then it was too late (all J/C class seats gone on UA47)
This does NOT make sense. If there has been no valid ticket, why would UA be willing to give a seat (albeit in Economy) at all?
First the lady said I am not in their system because LH did not create a new ticket for UA, second that I am actually on the next day flight, third that she would put me on waiting list and lastly gave me an economy seat.
UA wont do that to someone, who had no ticket at all.