I was wrong again! 

Thank you for telling me what's going on with you.  I spent some of the night thinking about you and about our Nantucket house.  I accept the foreshortened bulkhead as a challenge to ingenuity and inventiveness. Here's how I would proceed: 

a) Test that with my socket wrenches and handles, if necessary pneumatically assisted, I could loosen - but not remove - the bolts with which the existing bulkhead is tethered to the concrete base.

b) Make a list of lumber and of hardware which I needed for the project, to determine whether those materials should be bought on the Island or brought over on a hand-truck.

c) I would have all required lumber and hardware on site at the house before removing the existing bulkhead. 

d) Using 1" or 1 1/2" carriage bolts, I would bolt pieces of precut 1/2" TREATED plywood or 1x8, 1x10, or 1x12 TREATED lumber to the walls of the new plastic bulkhead extending the walls of the plastic bulkhead to within 1/2 inch of the abutting side of the house. I would then strengthen the wooden extensions of the bulkhead walls by bolting multiple pillars of 2x10 or 2x12 treated lumber to the outside of the wooden extensions.  

e) Whereever possible, when attaching wood to the plastic bulkhead, use one or morr clamps to hold the plastic and wood, while drilling through them together. Then with the clamp still in place, insert the carriage bolt with the unthreaded end biting into the wood. With both the wood and the plastic in place on the bolt, place a large flat washer and a locking washer onto the threaded end before screwing and tightening the nut.  

e) Assemble the new modified plastic bulkhead and bolt it to the existing concrete foundation.

f) Close the large deep opening between the wooden wall extensions, by by screwing precut sections of 2x8, 2x10, or 2x12 onto the pillars described in d) above. One can also use three sheets of 1/2" treated plywood bolted together to close the opening.

g) Use 1x3 or 1x4 strips of treated lumber to cover the cracks between the wooden bulkhead extension and the house.

h) caulk all sites of potential leakage.