The problems you seem to be facing when allocating/changing seats seem to be quite common for LX's online interface - so far, I have only tried to change seat allocations twice when flying in F, and in both cases the change wasn't reflected after saving, other than that the newly selected seat turned up as occupied after re-loading the seatmap (I managed to block every seat in the cabin after trying repeatedly). In both instances, I called the hotline who were able to help me, even though they weren't particulatly helpful.
In one case, I were even able to reserve one of the blocked seats with the hotline, even though there were issues similar to reserving the seat upon checkin (which was made at a LH counter). At first, the agent said that he had to change my seat assignment, as the originally reserved seat was already occupied by someone else, but in the end, the BP showed my originally booked seat. This is definitely something they need to improve, but until that might happen, the hotline is probably the safest option for seat reservations.