I don’t have the R2 or Hugo2, but currently have a PS and Mojo to try it out. I’ve used a P1 with the Mojo before in the past too.
1) yes JetEffect still works with the optical out. One thing I did find to be careful with was adjusting the bass - I find that increasing the bass causes the Mojo to step down in volume. Not sure if that’s just a Mojo quirk or if the strength of the rest of the frequencies in the optical signal decreases in order to provide the contrast of more bass (kind of like dropping a signal to prevent clipping). At any rate it isn’t a problem until you decrease the bass again and then the output of Mojo suddenly jumps up in volume!
2) volume control does nothing
3) not sure how to test whether it’s bit perfect or not? By way of comparison I tried how Mojo would sound using the optical out of my DX80, and actually they do sound a bit different. Thanks to Nayparm posting a teardown in the S thread we now know the PS uses a WM8804G spdif converter. Not sure what the PR2 uses but it might be different - I remember the PL was the first Plenue to support optical out above 192 kHz. The R2 manual says it can’t play DSD over optical out but doesn’t mention 192 kHz as a limit, so maybe it’s the same spdif converter as the PL? There are some teardown photos of the R2 on fccid.io but I can’t make out the spdif converter chip.
4) I’ve never timed the optical out battery life on the PS but I would have thought the battery drain would be less since the amplification stage isn’t in use?
5) the most important thing for you regards optical vs coaxial vs usb input may be the kind of files you want to play into the Hugo2 - I see that Hugo2 only supports up to 96 kHz over optical, but up to 384 kHz over coaxial and 768 kHz on microUSB. For the Mojo with standard 44.1 kHz files sometimes I find myself wondering if the coaxial on the Mojo is actually the best, but can’t be sure I’m not just imagining sound differences
Can’t speak about how the Hugo2 compares.
For all intents and purposes I’ve found the Plenues pretty handy as optical sources as the UI is straightforward and gapless works well, with the bonus of being able to EQ the optical signal. All of the larger screen Plenues have optical out so finding an older model 2nd hand could be an option, although as mentioned above the more recent ones seem to support higher res optical output than the older models.