Jerry,
I appreciate your skepticism. My background includes BS and MS degrees in Physics. Before retiring last year, I taught physics, hydraulics, engineering mechanics, and electronics at the college level for many years. During the 8 years preceding my retirement I was the Chair of the Science and Math Department at Cape Fear Community College in Wilmington, NC. My department employed about 50 faculty and served typically around 4000 students per semester. I am an applications oriented physicist and have taught the Statics and Strength of Materials courses that cover the design of these structures many times. These courses were actually not offered by my department, but, by our Engineering Department. Cape Fear Community College engineering students have participated in several competitive international engineering events (human powered submarines, remote controlled underwater vehicles) we have always placed first, second, or third and we usually win the secondary "best design" award. Not a bad performance, especially considering that MIT and CalTech are usually in the competition. So,...we know how to build mechanical "stuff".
What does the engine mount need? This mount should be designed as a truss type structure. In a truss structure the tubes or truss members should experience forces that are only either tension or compression. No bending forces are allowed. The four engine mount points must each be independently fixed in 3-dimensional space by tubes that transmit only tension or compression back to the firewall mount points. Fundamentally, this means that each of the engine mount points must be contacted by at least three tubes. In this instance the engine case cannot be part of the structure because the rubber bushings allow motion. The dynafocal type mount you mentioned is an entirely different type of structure because the engine mount points are connected together by the (fairly heavy) ring system. I've attached a marked-up version of your photo. We need tubes where I've marked lines. The yellow and red tubes will fix points "a" and "b". The lower X structure should consist of straight tubes that lie in a plane. I would replace the existing tubes with the green tubes. Finally, on composite aircraft, I worry about how "hard" the hard points on the firewall actually are. So, although this is probably optional, I would tie the four firewall mount
points together with the blue tubes. These would be tension members and could be made of 3/8" tubing. They would represent very little weight penalty and may save a major airframe repair at, say, 1000 hours or so. (guessing here) Of course, what I've penciled in may interfere with existing components on the firewall or with engine components, etc. In that case, some other tube configuration may be needed.
I don't know about Chrissi's engineering qualifications. I meant no offense. All I can say is that everybody makes mistakes occasionally. I am reminded of a machinist who worked on the 2nd shift at a textile mill who, when I was a kid, would help me with my "build it" projects. "Red" Thompson was his name; his favorite saying was: "Don't worry about making mistakes,
I've never made one I couldn't fix".
Aubrey