I have seen students failing to guess the meaning of an unknown word very often and this happens to me also.
Yes, strategies such as matching the meanings with the unknown words are advocated and 9 out of 10 rule in guessing from the context is observed
It would be very good because when students ask some questions about word meanings, teachers would answer more confidently and have a rich vocabulary.
You can teach better and have a rich vocabulary and be confident while using the language
Students may learn the common words naturally but it is better if the common words are emphasized in a foreign language class so that students learn better
No, vocabulary can be learned while one is learning other things or with the usage of language.
Dictionaries can be good enough but word lists can be helpful also in order to practice the common words