Hi Kent,
1) The analog input is intended to be used for recording (e.g. recording your vinyl to the hard-drive) via the built-in ADC.
However, you actually CAN connect another source to this input and set it up so that this source will pass through to the output jacks AND you can control the volume via the remote. In this regard, the Olive is kind of like a preamp (with only one external input).

2) I think the wireless G is the fastest you can go...
3) They should sound very close (playing the CD from the CD drive vs off of the hard drive), but I have found (as have some of my customers) that playing off of the hard drive (after ripping to WAV or FLAC) sounds the best.
4) After the mods, I am all about using the internal dac. I feed the analog output of my modded Olive directly to my Sig 70s. I control the volume via the Olive's remote. However, the modded unit's digital output is also fantastic and if you want to use an external dac, the modded Olive makes for a top-notch, off-the-grid transport! If you go with an external dac, you won't be able to control the output volume via the Olive's remote control.
5) Yes, you can trasmit to the Olive Sonata units:
http://www.olive.us/p_bin/?cid=01_02_sonataThese were supposed to hit the market this year, but I think there have been some delays. The Sonata has both analog and a digital output, and the last I heard, it was supposed to be fairly inexpensive (no more expensive than a SB3)...
6) The Symphony and Musica are the same unit, except for the size of the internal hard drive. The Opus is supposed to offer a better power supply, a different dac board (designed by Resolution Audio, with SPDIF input fed by tapping off of the SPDIF output signal.... so no remote volume control!), and a larger hard drive, and IEC power input jack.
I do not mod the Opus because I would throw away the upgraded power supply in favor of the battery conversion, I would throw away the different dac board in favor of my analog output mods, and my modded Symphony and Musica customers who want more HD space are buying external USB hard drives.
Best regards,
Vinnie