1) If you each have a separate set-sail passes (on-line check-in document) you can all check-in at different times.
2) It depends on the previous cruise, but if everything went fine with disembarkation for the previous cruise, they will usually start boarding between 11:00am and noon. Yes you can check-in, board the ship, and get back off the ship again to go exploring. Remember though that you must be back onboard the ship by at least 1 hour before scheduled departure. Also, you mention booze. Celebrity limits you to two bottles of wine per person and does not allow beer or hard liquor, so make sure you stay within those limits.
3) They will start accepting luggage for the cruise middle of the morning. You can check your luggage without completing check-in for the cruise. If you received luggage tags in advance, make sure you have them attached to the handles of your luggage before you hand them over to the porters. If you didn't receive tags in advance, you'll get your luggage tags from the porters. After you've turned over your luggage you can leave and go explore or proceed to check-in.
4) It depends on the line but most have very, very generous limits if at all. As said above, your biggest limitation will be storage (I tend to store my suitcases under the beds). Also, remember there are certain things that aren't allowed on cruiseships like irons, candles, etc.
Note: There are two cruise terminals in Seattle. If you are getting a taxi to the port, make sure you let them know you need to go to Terminal 91 (if the taxi tries to drop you off at the pier across from Pike Place Market you are at the wrong terminal). This pier is a little bit of way from things, so if you plan on exploring you'll need to grab a taxi to and from the pier - make sure to leave plenty of time for traffic on your way back to the pier because that 60 minutes prior to departure is a hard cutoff.