I'm still new and haven't explored some features yet, please forgive my ignorance on the subject.

I've noticed some profiles say "This account is inactive. You can Change your Username to it!" and then others say "This account will be eligible for clearing on..."

Am I missing something about the ones not yet eligible for clearing? The one I'm looking at says it will be eligible for clearing on a date that's already passed by several days (I probably wasn't stalking it for months or anything creepy like that). I'd very much like the username, but I'm clearly not understanding how this feature works.

Also, I'm guessing the answer to this may vary. Obviously the person has been gone for a long time, but there's still the possibility they could return. So, is using this feature frowned upon?

There's a bug with accounts not clearing when they should: Accounts not clearing

But no, I wouldn't say it's frowned upon to grab an available username. Once in awhile someone will come back and be disappointed that their name has been changed but they also get a free name change as a result, so.