O.k., I found out myself how to do the trick and now, I'll provide you with my knowledge ;-)

Discrete-codes were found on a RC5 basis for inputs and picture formats:

EXT1 (YUV) RC5 3, 4 / 5000 0000 0000 0001 0003 0004
EXT2 (SCART) RC5 0, 57 / 5000 0000 0000 0001 0000 0039
EXT3 (SCART) RC5 3, 56 / 5000 0000 0000 0001 0003 0038
HDMI1 RC5 3, 57 / 5000 0000 0000 0001 0003 0039
HDMI2 RC5 3, 6 / 5000 0000 0000 0001 0003 007B
HDMI3 RC5 3, 6 / 5000 0000 0000 0001 0003 0006
Front RC5 3, 5 / 5000 0000 0000 0001 0003 0005 (not sure/not tested)
Side RC5 3, 58 / 5000 0000 0000 0001 0003 003A (not sure/not tested)
TV (antenna) RC5 3, 72 / 5000 0000 0000 0001 0003 0048

Auto Format RC5 3, 61 / 5000 0000 0000 0001 0003 003D
4:3 RC5 3, 117 / 5000 0000 0000 0001 0003 0075
14:9 Zoom RC5 3, 80 / 5000 0000 0000 0001 0003 0050
16:9 Zoom RC5 3, 62 / 5000 0000 0000 0001 0003 003E
Breitbild RC5 3, 116 / 5000 0000 0000 0001 0003 0074
Super Zoom RC5 3, 126 / 5000 0000 0000 0001 0003 007E

Furthermore, there are the following working discrete codes:

Power On RC5 0, 63 / 5000 0000 0000 0001 0000 003F
Power Off RC5 0, 61 / 5000 0000 0000 0001 0000 003D

There is still the problem with the mute symbol, that is permanently shown on the monitor as soon as the TV is muted and connected to the home theater system. I created the following workaround:

When AV components (played back via an AV-receiver) are switched ON, I installed a chain of IR codes sending 40 x code for "Vol-" and, after switching the AV components to OFF, another chain of 25 x "Vol+"-command. Thus, I always reach a volume level of "0", even if before I had a little bit higher volume levels. And, at the end, I will always come back to my normal level of "25".

Hope that helps!

Cheers!