1(a) As long as it just seems as if you simply mixed up the dates due to human error, they will just correct your report. But if, say, you forgot about a five week trip and didn't report it, then that's a different story.
1(b) They shouldn't return it. See above.
2(a) see 1(a)
2(b) see 1(b)
2(c) Don't worry about exact wording. These are humans. They can read and think
3(a) as usual, as long as it's a minor change they won't care. But if you missed a whole job, that's a different story.
3(b) Employment history is only reported in months. Days are irrelevant. What you describe is not a gap.
4(a) this might, in fact be your actual problem. Please read the FAQ here:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=573&top=5
There it says: "You may send a transcript, diploma or certificate showing that you graduated from a secondary school or from a post-secondary program in Canada or abroad. These materials must show that the program was in English or French. A single course in an official language is not enough to meet this requirement."
If I understand you right, you only took some English language courses. But it is not enough to show that you took some courses. You must have graduated from a whole program that was conducted in English. So you need some kind of diploma/certificate that you finished that program.
There are other government-funded courses that count, but I couldn't find the courses you describe on any of the CIC lists for British Columbia. Look here in the instruction guide for British Columbia. Only ELSA is listed:
http://www.cic.gc.ca/english/information/applications/guides/CIT0002ETOC.asp#language
4(b) They won't frown upon you highlighting something. But again, I am not even sure if your proof of language is sufficient at all.
5 obviously "work permit" means you are a "worker". It's basically the same word
6 see above
General answer: You worry too much about these details. The only thing you should worry about is the proof of language thing. But maybe someone else can chime in. Don't freak out because I say that the proof you provide isn't enough. I might be wrong.