[Fanatical] Pathfinder: Kingmaker Imperial Edition -89% off

99% of games keys will redeem what you are missing... there are only a few games out there that do not.

Does this activate for people who have the base game to give them the DLC? [..] Worst case you're left with a shiny new key that someone else gets to enjoy...

I'm not really making any sense of any part of this. Is there a context I'm not getting? If not, do note that you're always able to activate any key, you just might need to go into your client's Help > Steam Support and remove your old key from your account; At that point, you'll be able to activate your new key without issue.

That said, the only time you should be running into issues with a key activation is when it's a duplicate registry, which generally only happens if you have 100% the same content or if the only difference in content is regional settings and/or censorship (and I haven't seen any instances of games using the latter settings in years, with most developers of those already scarce games having since just listed such games with entirely different Steam pages instead).

Perhaps you're thinking of in-client purchase issues, which occur when a publisher has misconfigured a package or bundle so that its store page listing doesn't list all of its available content. At that point, Steam inexplicably only checks the publically listed content, rather than the actual included content entries, and will restrict purchase based off that, if you own 100% of what's shown.

(For example, Life is Strange's full season package had such issues- iirc, it originally didn't include anything but the first free episode in its publically listed data [I believe since it was first made available when only the first episode was actually out, and hadn't been updated to include the later episodes on the store page since then, even despite the first episode having gone free]. As such, you couldn't enter without first removing the first episode from your account.)

In theory, if this occurs for a bundle rather than for a package, you may end up having to pay more, to replace the removed items. Other than that specific scenario, however, pretty sure you can always just remove existing content from your account without any penalty to your new activation. Though, again, you'll want to double check you don't actually own everything first (as, again, that's the usual reason activation will fail).

Yes, the Imperial Edition is everything (except the two free DLCs you have to add on your own). There are actually 4 paid DLCs.