Creative Mode With Achievements [DEPRECATED]

Due to mojang changing how Addons and achievements work this addon is now impossible to make and is now discontinued.

(From mojang)

If I use a mod that isn't from Minecraft Marketplace, does that also qualify for earning achievements?

No, only add-ons that are released via Minecraft Marketplace will allow achievements to be enabled. We do this to protect our players and avoid any mods that would allow you to unfairly earn a large number of achievements in a short period of time.