I flew to Halifax over the 4th of July. (I was using a $300 CO travel cert). I only fly one a year on CO for the last few years. I will do my best to answer your questions.

1. It is intl for checkin purposes. I had to check in at the counter at LAX even though I had no checked luggage. Unable to do web checkin. Some of the CO regulars can tell you if it is intl for upgrade purposes.
2. I cleared Canadian Customs in Halifax for EWR-YHZ.
3. Coming back, there is no preclearing customs like in Toronto, Calgary, etc. Return flight clears US customs in EWR.
4. I had long layovers in EWR in both directions. (at least my free Pres Club coupons came in handy). I cleared security at Halifax quite fast. I came 2 hrs before & I was bored. In my case, an hour was more than enough. The flight to/from EWR is in a RJ. They board only 10-15 mins before the flight if I remember correctly.

Note: Some of the more experienced CO fliers can give you more & better info.