While I greatly respect the knowledge and experience of those who have thousands of hours more than I could ever hope to get, I think this is being over-thought.
I built an RV-9A from a quick-build kit, and did the test flying myself, even as a low-time pilot. Was I concerned about such an endeavour? Not in the slightest. Did I consider reasonable risks & events that could occur? Most definitely. Other than some EMS alarms that were set a little too conservatively for a new engine, the flight test phase went off with only a few minor issues.
My RV-9 was the 1020th RV9 flying, and while each may be slightly different, the chances of any particular RV9 having such significantly different flying qualities from the factory demonstrator so as to make it hazardous are so remote as to be considered irrelevant in the risk assessment. My insurance had no restrictions on me carrying out the test flying either.
The vast majority of homebuilt aircraft these days tend to be similar in concept to my -9, in that they are not designed by the builder, rather the builder orders plans, or a kit, from a reputable manufacturer who has done the necessary flight testing to show an average pilot will not kill himself in that design through any fault with the design.
Now, if you decide to design and build your own plane, then professional help would strongly be recommended, but I do not consider it at all necessary provided you properly prepare both yourself and your aircraft.