Aww c'mon David. Your busting my balls.
I noted the 7 findings as a compendium of rationale for blast tubes. Not as the final justification of the choice for the P-47. As a lawyer, you should know that putting forth a defense is not formulated upon a single thrust in logical argument, but rather from a collective set of arguments that overwhelmingly influence the conclusion as being truthful based upon the preponderance of the evidence.
Same in reverse engineering. These 7 features of blast tubes are the result of different installations, their resultant problems, and identification of a single solution. Engineering consideratios such as metallurgy, kinematics, environmental, thermodynamics, structural, lifecycle support of the equipment, and human machine interface all drive unique solutions that may be combined in a composite approach to maximize common design goals.
The basis for blast tubes is no different. The P-47 offered unigue design challenges; of which the blast tubes afforded an ability to address a "composite" of P-47 engineering concerns. I doubt that you will ever discover a comprehensive list of the all the top down derived requirements that forced the blast tube incorporation decision in the P-47. Like legal discourse, sometimes decisions are driven by politics, fate, historical legacy, and even pure ignorance (ie the not invented here syndrome).
In my search to discover the above 7 advantages of blast tubes, I did discover that virtually all...read that ALL...fighters contained some form of blast tube installation as part of their weapons package. Check out B-25s, PBYs with forward firing guns located in the keel, F3Fs, P-38s, P-39s, P-40s, Boeing P-26 Peashooters, etc. Hell even the Polikarpov I-16 used them.
We can all make suppositions about which of the 7 fit the P-47 design criteria. The primary wing spar on the P-47 is located right behing the gun bay and 4 guns must be staggered to accomodate the perpendicular feeding belts from the magazines. This is the primary reason for the staggered design of the .50 BMGs and thus perhaps one (1) rationale for the exposed blast tubes. However, to come full circle, I will bet you bottom dollar that ease of maintenance ranks right at the top.
Take the word of a lowly aerospace engineer. My advice is free and is worth exactly what you pay for it.