1-yes and no, PRG-MUC-JFK and HEL-FRA-PRG without any problems, my bags were checked to my final destination, but on route EWR-FRA-PRG I had to pick up my bags in FRA and check-in again.

2.didn't notice special counters for sby pad in FRA/MUC, I went to normal economy/business cl. counter

3.If your ticket is routed via FRA and zones are same (or lower), it should be OK. Airport taxes in FRA are the highest in Germany, so they should accept your FRA-ticket in DUS,MUC,TXL,... as well.

4.Usually red smile means the flight is almost full, BUT the colour of smile is depending on booking classes availability (not real loads) - example: flight is oversold, but all booking classes are still open and tickets to this flight available = green smile on flyzed! The best way is contact Lufthansa office about checking loads.

5.Maybe you mean sby-waitlist screen. In FRA it is situated in the gate area, very easy to find it, there is info about number of standby pad, your position in the waitlist and your seat (if you are accepted) and information "flight is fully booked or flight is not fully booked".

6.It's all about airport staff. In case Y-class is full and J-class free, some frequent flyers (Miles and More card holders) will be upgraded to business and sby pad will be seated in economy - but if you are nice to gate agents and well dressed, you can by lucky and enjoy J-class. (btw. Lufthansa offer ZED business class to some airline employees)