Thank you!

Yes for windows you download the exe.

When I get home and want to install this on my Manjaro (Arch based) Linux box, I'm not sure what to do...

I personally don't run linux except for servers - and I've only tested it only ubuntu. Does one of the versions avaialble work on your linux machine?

As far as the passwords, are they only stored (encrypted) on my local box? or am I giving you access to them through the program? I think I already know the answer, but I'm seeing a few comments where a bit of clarification would calm their nerves...

Everything is stored locally - encrypted. Steem Messenger is not a service but a tool :)