The morning you board the Ranger, your 2 canoes will be "tagged" with red tags to indicate Mott. You will set your gear in large wheeled carts marked for Mott. At both Mott Island and Rock Harbor there is a water spicket conveniently located to fill your water containers/bottles in a jiffy.
At trip's end, you can board at Mott. The ship usually overnites at the dock at Rock Harbor. 9am departure, stopping at mott 9:18 (approx). The loaders at Mott will want your gear "staged"(ready to go) by 9:00am. Again, u will put your gear on wheeled carts. BUT you may have to stage your gear by 7:20/7:30AM (read below).
Sometimes (and there is no good way to predict it)(usually late in the season, but also weather-dependent throughout the season), the Ranger will overnite at Mott. In that case, you have to plan to arrive at Mott by 7 am. Loaders do their thing 7:30-8:00am, and then the liners (using the on-board crane) do their thing at 8:00. The Ranger will leave the dock at 8:20 or so, but you must have your gear staged no later than 7:30am. Who has responsibly stored the return tickets so they can be produced at trip's end to board the Ranger? (Always a challenge for me).
It will take 12-15 minutes to stage your gear once you hit the beach at Mott (100 yard walk at most). You don't have to organize your gear at that point; just get it onto the carts. If you end up waiting at Mott because the ship did overnite at RH, you can "brew" some coffee on the cement off the NW corner of the dock warehouse.
By all means, check with the boat pursar Paul Anderson on the sail over to the island, to let him know you will be exiting the park at mott (it will be written on your backcountry permit), to confirm that it's kosher. Even If your permit says Exit @ Mott, you are still free to board at Rock Harbor instead, and you don't have to inform anybody of that change.
The challenge in exiting at Mott is figuring out where you will camp the last night so that you can "make" Mott by 7 am. Caribou Island is the obvious choice, but it is popular with boater and sailors, and there is only 2 shelters and 1 tentsite (the tentsite is very close to the "upper" shelter). It is rare that i see anyone at the tentsite.
I usually do Daisy Farm, as the shelters are close to the water (i can load up quickly in the AM), but i get up early to do the 2.3 mile paddle to Mott. The lake (especially the harbor) is nearly always quiet that early in the morning. As soon as the sun rises a bit, mild slow rollers will appear on the harbor, so the early bird definitely does get rewarded.
It's a good idea to check with a boater, a kayaker (probably has a weather radio), or Ranger Rob @ Daisy Farm ranger cabin, to rule out a squall or storm moving in overnite. If it is rough getting back to mott, as Ingo said, keep to the shoreline and then cross the harbor when you are directly opposite Mott; if you then run into trouble, your group will be in plain view of plenty of park staff, getting ready to organize their day. If rough water is predicted for the AM, then your best bet is the tentsite at Caribou. And the luck of the draw may find you in a shelter there-one can get lucky, even late in the day.
By departing from mott, you won't have to set up (and TAKE DOWN) tents your last night in the park, as there are 16 shelters at Daisy Farm. There are 9 shelters at RH, but you're not likely to get one unless you arrive there by 1:00 or 2:00pm, even in mid-june. And it's a fair distance from the boat dock to the campground, and even longer distance from where you unload your canoes at RH to the campground.
The NPS doesn't "advertise" exiting the park from Mott because it's not straightforward (contingencies depending on where the Ranger overnites), and some parties would inevitably miss their return sail. If you decide to exit at Mott, be on time: the one time I was late, the crew didn't hesitate to let me know they were not happy with me. Now i'm sure to give myself a time cushion, and if i'm extra early, the coffee is brewing and that last bit of granola and dry milk is getting used up.
P.S. Since u will have 5 in your party, i suggest that on that final morning paddle to mott, have the odd-man-out hike 2.25 miles east along the level rock harbor trail; meet him/her at the Siskowit Mines (pretty much across the harbor from the Mott Island boat dock); then paddle with 3 persons the 1/3 mile straight across the harbor to Mott. Make sure the hiker has strong ankles, good boots and good judgment to avoid a sprain on this short solo jaunt! (I just got done with this very plan for departure from Mott on saturday, june 22!). Plus you get to tell some jokes at this person's expense while s/he is not in your presence. Though it is hard to take serious offense at any jausting on isle royale, as it's all part of the wilderness experience.