I blinked and missed a few responses :-)
1. For future, if you look through syslog you should see the adapters initialising both when the system starts and when tvheadend starts. An extract from mine, for example:
Nov 15 13:54:08 Server kernel: [ 20.217081] dvb-usb: found a 'Hauppauge Nova-T Stick' in warm state.
Nov 15 13:54:08 Server kernel: [ 20.217118] dvb-usb: will pass the complete MPEG2 transport stream to the software demuxer.
Nov 15 13:54:08 Server kernel: [ 20.217260] DVB: registering new adapter (Hauppauge Nova-T Stick)
Nov 15 13:54:08 Server kernel: [ 20.247264] em28xx #2: Identified as PCTV DVB-S2 Stick (460e) (card=80)
Nov 15 13:54:08 Server kernel: [ 20.247268] em28xx #2: v4l2 driver version 0.1.3
Nov 15 13:54:08 Server kernel: [ 20.252038] em28xx #2: V4L2 video device registered as video2
Nov 15 13:54:08 Server kernel: [ 20.252284] usbcore: registered new interfacedriver em28xx
Nov 15 13:54:08 Server kernel: [ 20.422450] DVB: registering adapter 0 frontend 0 (DiBcom 7000PC)...
Nov 15 13:54:08 Server kernel: [ 20.633939] DiB0070: successfully identified
Nov 15 13:54:09 Server kernel: [ 21.383119] Registered IR keymap rc-dib0700-rc5
... et cetera
and, later...
Nov 15 13:55:19 Server tvheadend[1673]: dvb: Found adapter /dev/dvb/adapter0 (DiBcom 7000PC) via USB (480 Mbit/s)
Nov 15 13:55:19 Server tvheadend[1673]: dvb: Found adapter /dev/dvb/adapter1 (NXP TDA10071) via USB (480 Mbit/s)
Nov 15 13:55:19 Server tvheadend[1673]: dvb: Found adapter /dev/dvb/adapter2 (NXP TDA10071) via USB (480 Mbit/s)
Nov 15 13:55:19 Server tvheadend[1673]: dvb: Found adapter /dev/dvb/adapter3 (Sony CXD2820R) via USB (480 Mbit/s), Reports valid SNR values
These would be good pointers as to whether (a) your OS sees your cards, and (b) whether tvheadend can also see them (although log format may have changed).
2. Re: wiki - Adam only merged the new DVB code into master yesterday, I think the attention is currently on "getting it working" rather than documentation.
3. Full mux RX mode is, presumably, the old "enable full mux reception". Unless you want a full mux dump, you can ignore that for the moment. There was an intention to use this to speed up channel switching between channels on the same mux, but I don't know if that's made it to the new code or not.
3. Keep FE open - pass, I'm guessing that it locks the DVB frontend (/dev/dvb/adapter0/frontend0) rather than letting it go, but I can't say why. My guess would be to prevent other applications from querying the device, or maybe to prevent a tuner from going into power-saving mode - although there may also be system limitations about having multiple devices open at a time (still guessing) so this might give performance advantages on single-tuner systems, something like that.