You really shouldn’t need to specify decorated names if you’re using the same compiler as the library you are proxying.

You can also probably proxy the system library version.dll which only has C linkage exports.

I made a template for this a few years ago here: github.com/SubstituteR/...

I’m not entirely sure that’s it as I’ve been on the WSL steam and normal steam at the same time, and I’m pretty sure they use IPC for a lot of things.

If you don’t mind either embedding a free (non-publisher) web token, or setting up an intermediate service,
GetOwnedGames
partner.steamgames.com/doc/webapi/I...
A caveat is that they would likely need to have their game history public. It may return friend-only history if the token owner is friends.

Would GetFriendGamePlayed suffice?
partner.steamgames.com/doc/api/ISte...

This runs from the steamworks sdk.

They can,

partner.steamgames.com/doc/webapi/i...

Under CheckAppOwnership