1989 Vanagon 2.1L  2-wheel drive 125k

Hello all - I have two issues:

Issue 1 - I have a leaking clutch slave cylinder and I am planning on
replacing it. The clutch master cylinder was replaced 10k ago. I have read
numerous posts on the archives emphasizing the importance of replacing both
slave and master at the same time. My question is...is replacement of both
master and slave at the same time only necessary when both have numerous
miles on them? Since the master is relatively new (10k), I was hoping I
didn't have to replace it. I know it is my risk, just asking. If anyone has
replaced only one with the other being fairly new and has information to pass
on, please do!

Issue 2 - I recently completed a cross-country trip (approx. 5000k) from
fairly warm northern CA to cold NY. I constantly checked my coolant level
during the trip (every stop). The coolant level never went down. Upon
arriving at my destination (Cold NY) I noticed the coolant level in the
refill tank was maybe an inch lower than usual. The next day I looked under
the engine and noticed a coolant leak. The leak was not bad, just droplets
forming beneath the engine. There was no noticible loss of fluid from the
refill tank during the time the van was sitting (3 days) so the leak was not
bad while sitting.  I thoroughly inspected the engine and could not find any
leaks. I inspected the rear heat exchanger and found no leaks at the valve. I
have not yet tested the expansion tank for cracks but will do so eventually.
Today (still very cold) I looked underneath and...no leak, no visible
droplets forming. I have researched the archives and understand that the cold
weather could cause problems with hoses, clamps, shrinkage of head gasket,
etc. I wish I could be more specific about where the leak is coming from, but
I don't have the facilities to do a thorough inspection. Mechanic is looking
at it tomorrow. Anyone have thoughts on this? If it was a head gasket
problem, wouldn't the leak continue in cold weather while the van was
sitting? My thanks to all that have replied to my earlier post and apologies
for repeating myself.