unlearnt words appear in the Words section and skill strengthening section.
unknown words are often added automatically in the words learnt section. These words are also later included in the strengthening tests.
This is true. It's just something we've all adjusted to. If you click on the word it should give you a drop down list of definitions depending on the context. I often pick the wrong one the first time myself. But, it's more familiar the second time around. I hope this helps! :)
PS Here are some lingots for a heart refill.
I believe that's coz the lessons have more words that we come across in two or three runs. The words in the Words tab are taken from the lessons/skills completed and not just limited to ones which the user came across.