I was also fighting the last days to get the 292e stick to work with my raspberry pi & tvheadend (3.9 from the unstable repository).
One thing I noticed is that the stick does not register two devices (DVB-T/T2 and DVB-C), but only one and there is no way in TVH to force the stick into a specific DVB mode. Per default the stick is in DVB-T/T2 mode. Thus, if you are searching or trying to play back DVB-C channels you will not have much luck.
I found a workaround:
1. Start "w_scan -tc" and stop it immediately with Ctrl-C. This will set the stick in DVB-C mode and leaves it there.
Alternativesly, you can execute "dvb-fe-tool --adapter=0 --frontend=0 --set-delsys=DVBC/ANNEX_A".
2. Restart TVH with "service tvheadend restart"
3. Now go to http://<yourserver>:9981 and check that the stick is enabled and so on
From now on DVB-C playback should work. I wished, TVH would have a dropdown to change between DVB-C or T/T2...
Another thing I realized. There are two different versions of "dvb-demod-si2168-b40-01.fw". The latest version 4.0.11 was not working for me (w_scan did not find anything). So I used 4.0.4, which just works.
dvb-demod-si2168-b40-01.fw:
http://palosaari.fi/linux/v4l-dvb/firmware/Si2168/Si2168-B40/
So far everything is working fine and stable. But for whatever reason I do not find all MUXes with w_scan that I do receive with another DVB-C receiver (they are not encrypted!). This is really bothering me, because some of my favorite channels are missing now :-/ I have a Sundtek Pro III on the way to me, so I will be able to compare soon ...
Best wishes,
Klaus