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As the headline says, are there any player scenarios available for editions of the game? There are a few on Steam already but I want some for here. Any idea where we can download them (safely)?
There's 2 or 3 at NexusMods, plus some mods, but that's the only safe place I know of.

You do have to create an account. I've read a lot of folks complain that you have to pay to download stuff from there but I have not had that problem, although they do try to get you to monetarily support the site I haven't found it to be mandatory.
Also interesting might be Vortex, it's a mods / scenarios manager which support Dawn of Man.