Can you actually make a KDF mission even if your Foundry dude is FED?
The KDF side is just too boring to try and reach that much dilithium

You can build the mission, but you won't be able to do the map test within the foundry. To playtest you'd have to publish test versions.

Or at least that's what the warning always tells me. I've never actually tested it

I haven't tried, but I believe you can. When you start a project it asks you whether its Fed or KDF allegiance.

That's accurate. I've done it.

To the OP: you don't have to level the test character. Just create him/her, and use him/her to test your maps and such. That's all.

Problem is they take up a character slot, right? Maybe the OP doesn't have a free one.

I believe some people only use one foundry slot and just create a character for that faction at the start of each foundry session. Doesn't mean it's true, but there it you go.

The thing to keep in mind is that you really should have a KDF character in-game of high enough level to play any mission you create for that faction. Things don't always work the same in the live game as they do in test plays within the foundry.

You should always play through a mission you create right after publishing so you can withdraw it if something is amiss. A couple of bad ratings right out of the gate because of an unnoticed technical issue can bury your mission for good.