Dental cavity curiosities?!

You usually need a root canal when the pain get unbearable from the nerve infection. Sometimes it's just a dull constant pain and sometimes it's a bang your head against the wall type of pain.
The main symptoms are usually pain and the tooth doesn't feel right. There might only be pain when chewing or drinking.
Yes a cavity can require a root canal. Sometimes fixing the cavity is the cause of the root canal especially if it's a bad cavity and the dentist has to drill deep. Sometimes they hit the nerve which will cause all kinds of problems. This has happened to me once.

Can't answer the last question, only your dentist can. He/She will need to fix the cavity and then you will have to go from there. Sometimes if it's so bad they will know ahead of time if you'll need a root canal but usually they will fix the cavity first.