[Pblais]

In a general sense your assumptions 1 -7 are mostly correct in the wide context you use though In the area of North Carolina offshore of Cape Hatteras is not close to shore. The Diamond Shoals extend a great distance to the east and a close shore passage could be exceptionally dangerous in anything less than very good weather. It's been sinking ships for 400 years and adds a few more once in a while. It is possible to go far enough off shore or go in shore via the ICW to avoid that section as you suggest in other points.

The Delaware River to the Chesapeake / Delaware canal is suitable for ocean going commercial ships. You can go in and out with no barrier. You can find more details about the passage. There is no connection north of Philadelphia to any place you might care to connect with. You have to go offshore from the Delaware River to Sandy Hook, New Jersey.

With 4ft of draft and a 40 ft mast you can transit the ICW with ease almost every place. None of the canals will handle that much clearance but facilities to unstep and re step the mast are handy.

8.I understand that it's feasible to transit via river/canal the whole way from Lake Michigan back down to the Gulf of Mexico, would this route require the mast to be dropped again; alternatively can you get from Lake Superior (via Minneapolis?) into this same inland waterways system?

There is no connection from Lake Superior to the Mississippi River other than the US 35 Freeway system and your boat would require a trailer to make the trip. The river actually flows not all that far from the lake but it's not really a river you could sail and is in now way connected to the lake. The St Croix River is also close but not connected or Navigable in much more than a canoe (great canoe trip!). To exit the Great Lake with a mast up there are only two possible points. One is the Illinois River in Chicago that flows to the Ohio River and then to the Tennessee / Tom Bigby canal or on to the Mississippi River. The other is the St Lawrence Seaway to the Atlantic Ocean out Lake Ontario. The entire Great Lakes can be transited by the very large commercial boats end to end and so can you. There are some locks but there is sufficient vertical clearances. Only the canals require mast removal.

9.Whilst we're not obliged to hold US Skipper qualifications to sail offshore, is there a separate qualification for inland waterways and would foreign flagged (British) yachts need to comply

So long as your boat can pass USCG regulations of safety equipment you can be anything you like to claim and you personally will not require any personal license or demonstrated qualifications. Customs and other assorted paperwork is a different matter.

10. Last One for now - How far North do we need to go up the east coast to reach a boatyard/dock that's clear of the 'Hurricane' zone and/or would we be able to moor/leave the boat somewhere further south if it were on the inland waterways?

The North Carolina / Virginia border is considered traditionally to the the line where your insurance will allow you to travel and be covered. Your own insurance situation would be the better authority as far as how far and on what dates they apply. As far as leaving the boat further south. You may do so any place you choose. I suppose it just comes down to what you choose. If you haul the boat out it might be most any place. Marinas north of Virgina will be much more expensive since the ratio of spaces to boats is drastically lower. In New England availability at even expensive prices can be a problem as well.