i have two 522s, and one has substantially higher trigger voltage than
the other. i don't remember the exact voltages but it was something
like 10v in one and 100v in the other. i was rather surprised at the
difference. to be safe, get a wein safe-sync.

it should work fine with x sync, and you'll just get the auto settings
from the flash. the only custom 'module' i've seen for the 522 is the
remote unit which is for olympus.

they'll work but you'll need to stop down meter if there's no 'a'
setting on the lens, and obviously, no autofocus either.

Not directly.  The *ist-D has a PC socket, but this is one of the
features that was left off the *ist-DS.  You can, of course, get
hot shoe adapters with PC sockets.

Yes.  The functionality you get depends on just how old the lenses are.
Anything with an "A" setting on the aperture ring (i.e. just about any
lens made in the last 20 years) will work just fine on either DSLR.
(You leave the lens on the "A" setting, and the body controls aperture).
With the really old K-mount lenses (both the original K series, and the
later M series lenses introduced with the ME and MX) you are restricted
to hyper-manual exposure mode.  This is basically manual exposure mode
where you set the shutter speed and aperture directly, but with the
extra convenience feature of a one-button way to tell the camera to
select the shutter speed based on the currently-selected aperture.
This is done using stop-down metering; Pentax are moving to body-
controlled apertures, and on the DSLRs they have left off the coupling
necessary to read the position of a lens-mounted aperture ring.