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It is possible to do stereo recording? At least, there are unsoldered pin's for stereo microphone on PCB?

[jOERG 2009-02-23] There are two testpoints H-TP3001/2 or (on later PCB revisions) 2 pcs 0R resistors for LINE1/2 input.

Output at Headset and Speaker at the same time

Is it possible to output some audio stream on the headset and in case of an incoming call to (optionally) lower this output, and additionally output the ringtone on the stereo speaker? Or is this not possible due to hardware limits (guess this would require full-duplex output)?

Look at the block diagram for the wolfson codec. It looks like this is not possible. You could eventually do it by using a bluetooth a2dp headset or with some hacking with a bluetooth voice headset.
[jOERG 2009-02-23] output to headphones and speaker is mutually exclusive. See schematics p.6 "SPK LOCATION:41XX". When you switch to speaker mode, EP_R will be muted while EP_L gets signal intended for speaker. Both speaker (aka "stereo"speaker / sounder / speakerphone-out / ringer speaker) and headphones share same ADC and path incl level controls associated to LOUT1/ROUT1, so you can't operate them independently. Of course it's no problem to do any mangeling of audio like adding streams via ALSA DMIX plugin or leveling down some (MP3) audio stream by using ALSA softvol plugin, at digital side prior to DAC conversion.