Upright and inverted can be both made to "Balance"... but making both "Stable" is perhaps more evasive.

rmh, you suggest insensitivity to stab height on the fuselage affecting Yaw-Pitch coupling.... how would you explain every
IMAC plane these days having lowered stabs? How would you explain the centrally located stabs on every pattern plane I have seen in the past 20'ish years?
How would you explain that the yaw-pitch coupling occurs in all flight conditions at all speeds? Well... at least with every plane I have owned.....

I dunno what your flying technique is like... if you flew faster pattern planes back in the day... becuase the beta never got high.. the coupling may not have been pronounced... I enjoy flying 3D and frequently get my GS planes in KE's with a Beta of 45 degrees... and higher.... this is particularly where the coupling rears its ugly head.....


Biplanes??? I can't 'splain bipes.. I had a 42% ultimate that did OK KE until it got to a beta of about 42 degrees.. then it pitched so bad no amount of back stick would rescue it.....

Also... in an earlier post, I suggested that CG-Trim combo didn't affect KE Pitch...well... I meant if all the other factors are small... then the CG-Trim effect is notice'able.... (But it isn't really yaw-pitch coupling as the plane pitches regardless of yaw or rudder)
Essentially.. all these phenomona exist and can occur.... but if everything is done to null the effect EXCEPT locate the stab properly... the coupling is likely to persist.

My Eclipse for example... the stab location is very close to the Null spot.... so the CG-Trim situation is the predominant force... in KE..... irregardless of speed or beta or rudder...