Steam – Activating another country’s key on Steam in United States, any problems

Yes, you can trust it. It is non-intrusive and all it does (at minimum) is manage your games and patches for you. Anything else is optional.

Steam has an offline mode, where you can play your games without a connection to Steam, granting you total freedom from Valve's servers.

It doesn't install anything but the software you need (like DirectX and drivers for example) and the games and patches. Nothing you don't want, unless you don't want the game you're installing. The only downside is that these patches are kind of forced on you, even for single-player games. If the developer decides to 'downgrade' the game or make a change you don't like, there's not much you can do about it.

I'm not aware of any 'independent verification', but the DRM Steam is itself is very harmless, you can choose to run Steam in offline mode and there would be absolutely no way for them to pull access to your game, except for the fact you wouldn't get patches. Any other DRM can be chosen to be used by the game itself of course, but that has nothing to do with Steam.

Anyway, the choice to use Steam has already been made by Square Enix. If you don't trust them, why are you installing the game itself?

I agree with corroded that you should just give them the key, but to answer your question:

If you purchase a copy of a game you already own, you may or may not receive a giftable copy to give to someone else. According to Valve, the only cases where you get a giftable copy are:

1. Users who own Left 4 Dead and then go on to purchase "Left 4 Dead + Left 4 Dead 2 Bundle" will receive an extra copy of "Left 4 Dead"

2. Users who own Half-Life 2 and go on to purchase “The Orange Box” or the "Valve Complete Pack" will receive an extra copy of “Half-Life 2”.

3. Users who own Half-Life 2: Episode One and then go on to purchase the "Half-Life 2: Episode Pack”, “Valve Complete Pack” or “The Orange Box” will receive an extra copy of “Half-Life 2: Episode One”.

4. Users who own Dawn of War II and then go on to purchase "Dawn of War II Gold Edition" will receive an extra copy of "Dawn of War II"

5. Users who purchase the available four packs for specific Steam titles will receive three extra copies of that title to gift to friends.

6. Extra copies may apply as a part of some limited time promotions. In that case, information on what will be included with your purchase will be listed on the storefront's transaction page.

When trying to activating a Game you already own (or a bundle you already own parts of), Steam will warn you in a popup which game/parts won't be added to your library. You then have the choice to cancel the activation.