Grimrock

Do I have to install Steam to be able to track LoG achievements?

If I do install Steam, does it track the possible achievements I would have already unlocked prior to install?

If I do not install Steam, are the achievements unlocked anyway, and just not visible to me?

If you install the game via standalone installer, you will not get achievements.
If you install the game on steam, it will track achievements but you will need to be online to play.

Whether or not the achievements get unlocked on steam if you played the game non-steam previously, I cannot tell. Some games can do that but that isn't the rule.

Achievements are only in the Steam edition.