May I know the state of the relationship between "lua" and "terra" (http://terralang.org/)?
If I've understood it right, "terralang" is currently an independent project, which raises the question of if in the future there's any chance that "lua" and "terra" might get merged!
Not a chance. As you say, it's an independent project. Lua is an embeddable programming language interpreter. Terra is a compiled language that uses Lua as a frontend. They serve completely different purposes and they're maintained by unrelated groups. Merging them would be a disservice to the vast majority of Lua users.