Not experienced enough to address all your points but here is what I have got.
Can students add entries to this glossary?
If they have been given access then yes.
Yes. The way I have been doing it is by having secondary glossaries for each unit and then pushing them to the main one. This way the students are not bombarded with a mammoth list. One thing I have not been able to figure out is how to restrict a word list and not have it show up in the main one. Currently, I am forced to do little things like this manually which is tedious work.
Can entries that were made in a course, later be imported to this glossary?
As I understand it glossaries can be used for any number of things including the things you have mentioned. There is no reason why you could not have them all and even have them under the main glossary or separate.
What is the relationship between a courses main glossary and making spelling / vocabulary lists?
I myself am also fairly new with Moodle. So far I am not using categories but that may change. My workflow has been to create a secondary wordlist for each unit. Making the list in Libre office (calc) and then importing in bulk saves a metric ton of mouse clicks.
Do I create a separate physical glossary for each word list (spelling list) or do I just create a category?
I have also been trying to do this but not found an effective way to do it. As there is only room for the concept and definition I have had to make to within those confines. Maybe I should add keywords, I just have not had enough time to try it out.
Summary: I am teaching an English class. Each week the students have a weekly word list. How do I organize this?
I also confused the keywords. Do Glossary entries only allow a single keyword? I would like to organize the glossary by
Part of speech
So far each entry looks like the one below. It is not perfect but it is good enough for now.
Controlling everything and forcing people to obey strict rules and laws.
The lives of the citizens were strictly controlled by an authoritarian regime.
If I decide to use "markdown", does the glossary allow me to use categories as mediawiki uses them?
Hope that helped.