...My question is, with the short distance, would I really benefit from ladder line at 2W with a balun just inside my window...
It is unlikely to make any significant difference.
The losses through 50' of RG-58 coax are less than 1dB. While the losses using open wire line may be
less than that, you'll also need to add a balun and a tuner, both of which can add losses that, while
small, may eat up much of the potential savings.
But changing to open wire line would also require that you use a tuner, compared to the simplicity of
direct coax feed without a tuner for the existing installation. Simplicity is good!
The situation would be different if you were using a single doublet on multiple bands, however.
... and if not should I bother with a balun on the 20/40 antenna at its feedpoint?
A balun might not make any difference. Or sometimes it may.
What a balun does is make antenna performance more stable and predictable
the interaction between the antenna and the length of the coax (and what is connected to it.)
Sure, I've put up a lot of dipoles without using a balun, and most of them seem to work fine.
My guess is that 75% - 80% of the time a ham won't notice any problems that they attribute
to not using a balun. But I've also seen a lot of quirky problems, some of which hams don't
realize are due to lack of a balun: resonant frequencies that don't change then the wire length
is modified; extra noise picked up from electronics in the shack due to the coax acting as part
of the antenna; changes in SWR due to "minor" changes in the shack, such as changing the
cabling or plugging in headphones; RF shocks off exposed metal parts of the rig, mic or key;
different SWR when measured by the rig vs. an analyzer; etc. In a couple cases adding a balun
DX performance by reducing vertically polarized radiation from the feedline
of a low dipole.
you bother adding a balun to your existing antenna? The choice is up to you.
If it seems to work well enough, and your receive isn't limited by noise pickup from the shack,
then there isn't much reason to. If you often change your shack around and find that the
SWR on the antenna changes when you do so, or experience other quirks, then it may help.
My backpacking dipoles use RG-174 coax with no balun. Most of my other portable dipole kits
don't use them either. But in one case we were able to reduce the noise level from S9 to
S4 by adding a balun, and in other cases it has eliminated problems from "RF-in-the-shack".
Sometimes it makes a difference, other times it doesn't.