So, experts, am I looking at a rebuild or engine replacement?  The beast
runs great with no indications of low oil pressure.

  I would not be happy with those numbers and sounds  from a motor.   I
don't have any WBX experience, but in most other motors, any type of
knocking under a heavy load is an indication that something is not right.
It can be hard to tell whether  your engine is having detonation or the
bearings are worn...they sound sort of similar...but with 20psi oil
pressure under load and 5psi at idle...that is 'low' oil pressure where I
come from....again, not WBX-land, but in most other motors that is
considered kinda low.

  It's pretty stressful sitting behind the wheel of a laboring vehicle that
is making bad sounds, knowing you have another ten or so miles of uphill
and wondering if you are going to make it..watching your mirrors to see
half a dozen cars and trucks built up behind you////

   Having an aftermarket oil cooler on your rig...that *should* rule-out
the oil getting too hot...but I'd check closely on that installation...when
similar stock vehicles work OK without a 'performance enhancer' but yours,
with an aftermarket improvement is getting hot...you may have something
going on with your installation.   Having the additional cooler and still
seeing 5psi hot oil pressure at idle...that can't be very good...20/50 oil
is pretty thick stuff.

   The good news is Spokane is relatively flat, you could drive around in
that valley without stressing your vanagon...the bad news is iin every
direction from Spokane you will be encounter big mountain passes.
 Having a Vanagon that causes you worry and stress when you use it,
especially when using it to leave town......that kinda defeats the purpose
of them.
 Having one that lugs down to 40mph  with knocking sounds and overheating
at each major 'hill'...If it were me, I would want to do something about
that.     It would be inconvenient to need to stop on big hills to let the
motor cool off all the time..It would also be inconvenient to blow up in
the remote areas of the northwest.
 It doesn't look enjoyable (to me, anyhow)  running the Interstates, being
passed in the truck lane by semi trailers with their flashers going...I
have seen quite a few Vanagons in that situation..