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.
That is the way it should work, but it seems it doesn't. I have words that haven't appeared more than once over a month ago. Whereas words that you fail at least once tend to come back.
Have tried practice weakest words again and it seems to grab the words you have seen least with a couple of really weaker words included. Try it yourself, you can ordering the words on last seen and strength tab.
Yes, it tries to do it, but it seems that there are some issues. What I noticed as well were some words that I had not seen for a long time, but it appeared in the list like I had seen them 2 minutes ago. A bit strange, I go take a look again.
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.