I'd add a couple thoughts to the discussion as follows:

1. In doing mechanical design a best practice is to develop several "competing" concepts for evaluation. In so doing one opens up the mind to more ideas. This prevents going down a single path which may have flaws and we spend all our time trying to engineer out the issues where a different concept may not have those issues. In the design as drawn I would be concerned, as I believe Rich is, about athwartships load components on the shackle/chainplate joint as the boat yaws. This is cyclical fatigue loading and the shackle/chainplate joint performs best in loads which are along the axis. I would be looking at connections which transfer load more uniformly stress-wise.over a range of directions of the load vector.

2. Generally in bolted connection design fewer larger bolts is better than many smaller bolts. Reason is that there will be dimensional variations in components which can result in uneven distribution of load among the bolts and can lead to premature shear failure in one or more bolts, weakening the overall structure. A prime example of this is the connection between a large jet engine and the wing spar on an aircraft like the Boeing 747. These large masses are attached to the wing spars with relatively few very large bolts.

just some thoughts.