Here's the follow up-
As intended, I ordered up some new 7/16 stainless bolts, washers, and nylock nuts (with wax) from Bolt Depot. They didn't have 7/16 nylock nuts in brass, so I went with stainless- not my first choice, but I think it'll be fine. In the process of researching, I learned that stainless on stainless threads are very prone to galling.
For those interested, more info on galling
here and
here.
The right-angle mounting bracket is to be secured to a piece of fiberglass, which I originally thought was sheet metal. This worked to my advantage as I had to run a drill bit through the existing holes to open them up ever so slightly to allow for the new bolts to be used; fiberglass drills more easily than sheet metal. The difficult part was simply getting a combination of wrenches, sockets, and a small impact driver in position to secure the fasteners. It took about an hour of trial and error, cuts and scrapes, body contortion, and some colorful language, but I was finally able to get the fasteners torqued down.
The reward- no more play in the steering! I also was able to observe that when I had the fasteners only hand tight and had my helper turn the steering wheel that because the rod coming out of the end of the steering cable sleeve is attached to a pivot point on the rudder box arm, it caused a side-to-side twisting motion at the bracket. Knowing this, I then understood that if only one bolt was present in the bracket, that torsional force would push and pull on the bolt and could likely cause the washers to cave in and give way.
The bonus discovery was that I found the brass nut rolling around under the location of the missing bracket fastener. AND... it was NOT a nylock nut! Though it was brass. As far as I'm concerned, the fact that a non-locking nut was originally used in this application was the root-cause of the problem here. I have all the dealer service records for the boat, none of which specify any work ever having being done with the steering system. Makes me wonder if it was assembled at the factory like this, what was that person thinking using one locking nut and one non-locking nut on this bracket.
One final note- while doing this work, I noticed that the motor mount in this same location was about 1/4 of a turn from being snug- it was just loose. The boat was hauled from Indian to Maine last year when I purchased it. Perhaps riding on the trailer all those miles shook a few things loose. I'll be inspecting every fastener I can find on the boat before she goes back in the water in a couple weeks!