1) Yes done, but doesn’t mean that Security is done - that one you can’t see on the tracker.
2) Yes, they check first the PA (primary applicant) and only afterwards the dependant. You might be also of different nationalities which also can trigger delays.
3) Background check is mainly eligibility and then once it’s passed, they start Security. It’s authorities checking you in terrorist databases and with intelligence services - normally this check can last 1-2 months, but that doesn’t mean that once it’s done that there’s an IRCC agent available to put a pass on your profile - things can be delayed.