Due to the Qatar Business Class Sale I'm flying OW QR from ARN-DOH-CBR and back SYD-DOH-ARN in Business.
Now I need a connecting flight from FRA to ARN and on return from ARN to FRA. There is no OW direct flight, so I need to book *A SK or LH (Eco). I'm traveling with max. 1x23kg.

Is it possible to interline the baggage from LH (*A) to QR (OW) flights? Can LH check in my luggage to CBR directly?

If it's not possible, this will be my first time with a connection flight not on the same reservation. So what will be the procedure?
Get the baggage at ARN, go through customs and then check in to QR flight? Or is there any transit possibility at ARN?
I'm planning to arrive at 6:10pm at ARN and the QR flight departures at 10:40pm. So I think 4hrs 30mins should be enough (if there is no delay of course). On return I will arrive at 1pm and LH flight will be 3:35pm - so 2hrs 35mins. Will this be still enough?

Thanks for your help.