A high wing plane has a drag couple that plays into knife edge flight. Stick planes are often pretty good at aerobatics, but not so good at knife edge pulling hard to the canopy if they had one for the same reason as the Decathlon.

I'm not sure the inverted 45 deg upline test is a good indicator of proper CG for the Decathlon nor is inverted comparison with an elevator trim that is out of neutral.

Personally, I would balance it on the prescribed CG... then trim the decalage for neutral elevator... then re-balance for proper glide slope.

Are power on/off trims now the same... I ask because often when significant elevator trim exist, they are not.