2013 Subaru Crosstrek transmission issues

Over a week ago, while driving my Crosstrek across country from Colorado to
Florida, my At Oil Temp light started flashing (along with AWD, traction control,
and hill assist, I believe- but those were just lit, not flashing). I googled, waited
40 minutes on the side of the highway in 97 degree heat, turned car on again
and the light was still flashing.

I had my car towed to the Macon, GA Subaru dealership. The next day they
replaced the valve body (under the 100k extended CVT warranty) and sent me
on my way. About 50 minutes later, I had pulled off at an exit and was in a
drive thru lane when the lights came on again. I called the dealership and
payed a hefty tow fee to have my car taken back. The dealership said the code
was actually a different one than before and they needed to get my car to
replicate the error.

Now, over a week later, the dealership tells me they have put about 60 miles on
my car trying to get another code to pop up, to no avail. It seems they are
unwilling to investigate further. I am concerned that  my car has simply not
been driven enough at one time to replicate the error, but freely admit I know
nothing about cars. The first instance occurred on my 4th day of driving across
country, and I had been driving for hours prior to the lights turning on. The
second instance had been an hour of highway driving at 60- 80 mph.  

I am currently 4 hours away from this dealership. I am going to have to get a
one way car rental to even retrieve it. I am terrified of being stranded on the
side of a highway again and having yet another expensive tow cost I cannot
afford (I just graduated in June and have not worked in a year so I am living on
savings until I find a job).

My questions to this group are- 1) Should I drive my car at highway speeds in
the area close to the dealership for an hour or so before actually starting on my
4 hour journey home? 2) Is the heat of the day possibly a factor? Should I drive
in the evening/early morning and take frequent breaks for the engine to cool?
Or is that difference negligible? I really don't know what to do and I'm so
frustrated by everything and scared to get stranded again.

Other details- car has 82k miles on it. All services have been at Subaru
dealerships and on time per Subaru guidelines. My engine was replaced by
Subaru at 62k miles (they covered it under warranty though I was technically
almost 2k miles out of warranty) due to a rod knocking and I was told by the
service worker managing my case it was a fluke given the service records on
my car. I have really started wondering if I just have a lemon on my hands.