Practice least recent words
Since it seems that words that you get right the first time (e.g. by guessing in multiple choice question) may well never appear again since they never decay (well not in six weeks at least), it would be good to have the option to practice words that have not appeared for a long time, similar to practice weakest words.
From what I experienced is that when you practice weakest words, even when al the words appears to be strong, only least seen words over time will be used. If you practice again, then the next set of words will be used. I think it is some kind of roulette ordered by date. If you're sure that you will never forget a word (like "Hello") you can exclude it anyway. I think that when you keep on practicing, the word you forget all the time, will stick eventually.
I agree that it would be very beneficial to practice least recent words and I hope that's developed. In the meantime, I go in to each lesson and practice weakest words and it seems to pull from the bottom of the list. Not exactly efficient, but I try to keep all words within 2 weeks of viewing/usage. The other good thing is that I can avoid some early lessons in which the words are no longer challenging.