Should I worry about attenuation for the
HF bands enough to use the RG-8 over the RG-58??

Without some experimentation, it would be hard to say definitively, but I'd say
not to worry about it much. If you can get the RG8 cheaply enough, great, but
it's stiffer and more difficult to maneuver around walls and fences, etc. If
that's not an issue for you, use the bigger stuff, but for a Sat800 and a
listening only situation, you probably wouldn't notice the difference.

Are those Alpha Delta Transitraps
as good as advertised, or should I stick with a regular gas
discharge variety??

USe what you can afford, but ensure a good connection to a ground rod. The
Transitrap units do work very well, but no arrestor will do good against a
direct strike (a handful of personal experiences notwithstanding). The
gas-discharge types are therefore no less advantageous in my opinion than the
Transitrap. Detach your lead-in from your radio when anything threatens, and
the difference between the two types will be negligible.

I'm planning on running a random wire until I can figure
out a more permanent location for a trap antenna that I'll
build

Since you're using a Sat800 in a RX only circumstance, you might wish to
consider, if space is no longer a real problem, to avoid trap antennas and
stick with whatever length of wire you can get outside and as high in the air
as possible. Traps can be lossy and if cheap can introduce noise into the
system, whereas a decently erected randomwire will reduce noise if the coax is
grounded (referring to another question, you should ground your coax for noise
purposes at the very least. Plastic case or not, it helps reduce noise.
You're using a nice radio there, and it's about as good as most other radios in
its price range (and better than some) but it's not crucial where antenna
experimentation is concerned. I'd say avoid traps, ground the coax shield at
the receiver AND antenna ends, and enjoy a more noise-free set up.
MY opinion only, of course. Tinker a lot, it's a hell of a lot of fun!
Linus