Just tune the highest frequency antenna first
and then work down..........

It's much easier if you start low, and go up...
If you have interaction, it's almost always a
"higher" band. So if you start low, you'll only
have to do each, once...Going the other way,
can have you chasing your tail after a while...
IE: the higher bands will usually change when
adding the lower bands if the wires couple...
Rarely the case, the other way around.
I've used parallel dipoles for years...
My favorite config for everyday use dipoles.
Once, even had paralleled turnstiles for 80/40 on
one coax...
At the present, I have 160/80/40 on one coax..
To make a noticable difference over a dipole,
you'll need to get some serious gain...If you don't
get at least 4-5 db over the dipole, I hardly think it's
worth the hassle...That G5RV would have appx 2-3
db broadside over a 1/2 dipole, assuming you aren't
losing power feeding it... I agree with Reg, I'd either
run coax, or ladder line, but not both....To me , running
both is an exercise in sillyness...But to many, their own...
Ladder line, if you have a tuner, or coax, if single band,
or a remote tuner, etc...

..................................................
Is it true, though, that when the ladderline/windowline gets wet it
negatively impacts the efficiency of the transmission line?
.................................................. ...

Depends on the ladder line, and whats in between the wires...
300 ohm line can be bad, and also other lines that have no
"windows", etc between the wires...Supposably, the "windowed"
ladder line is less effected by rain. Open ladder line with insulators,

almost no impact at all...I've seen 300 ohm line turn near useless
on UHF when raining hard. I used it to a large UHF TV antenna I
used to receive ATV. Great when dry, but get it wet.....
MK