When I go to close a thread, the option says "Close Thread (forced for archives)".

The "(forced for archives)" worries me; I want to close the thread, but have it remain in the game evermore for reference.

Is that what will happen?

Yes.  Archiving has to be specifically selected, and will automatically close the thread.  Closing a thread does not, however, automatically archive it.

Also note that archiving a thread does not remove it from the game, it simply makes the whole tghread (including private messages) visible to everybody.  If you want to save a thread outside the game, you need to use the D/L link to format it, then save it to your own computer.

Phew, thanks for that.  :)