I recently had the coolant system flushed and coolant replaced in my '91
Westy at a local independent VW service place.  I have about 15,000 miles on
my rebuilt (GoWesty 2.3 liter) engine.  When I got the vehicle back, the
coolant in the reservoir (non-pressurized tank) was at the 'full' line.
After driving the vehicle about 100 miles, I noticed the heater didn't seem
to be heating fully.  After I opened the heater valve 100% for a few miles,
the heater function seemed to be restored, providing well modulated heating
over the full range of the heater valve settings.  Engine temperature - or
rather coolant temperature - was normal the entire trip (consistently
mid-point on the coolant temp gauge, just as before the coolant flush).

However when I got home from the trip (about 500 miles, including some NW
Washington mountain passes, I noticed that the reservoir was empty (didn't
check the pressurized tank, alas).  I refilled the reservoir and after
running the engine a few times during short day trips around town, both the
reservoir and pressurized tank levels are normal (pressurized tank is full
and the reservoir is at the mid-full mark).

There is no evidence of leaking coolant - anywhere.  My guess is that the
air was not fully purged from the coolant system following the flush/refill
operation (perhaps, at least in part, due to air trapped in the heater lines
and/or heater core).

Two questions:

1)      What do I need to do (if anything) to ensure the system is fully
purged now?
2)      What possible adverse effects might there have been from having an
incompletely purged coolant system on a drive like this (including some
full-throttle periods over the mountain passes)?  In particular, of course,
is it likely that I might have damaged or otherwise compromised those
infamous waterboxer cylinder heads??

Also, a corollary question:  I clearly specified that the replacement
antifreeze must be Volkswagen brand only (in the past, I've had shops use
'house brand' coolants - i.e. whatever the local tech/guru thought was
best).  How can I verify that the antifreeze used was genuine VW and not
some off-brand?

-Wes