Ok, so here’s the latest ‘result’: Unique Download Link | WeTransfer
This time I was extra careful when re-doing the calibration at home, which led to AHRS_TRIM_Y,0.001047045. (It might be that I was a but misled by the curved shape of the fusilage when I had done it last time, this time I only took the tail boom for reference.)
On boot I also made triple-sure the plane was level. All in all I took more care with the levelling than I ever had to do before with any of my planes and copters. I didn’t change anything else in the config, by the way.
But instead of being more stable, the T1 was more or less uncontrollable, it tumbled through the air, and on the last attempt, even flipped over right after launch, as if stabilization was suddenly inverted. 
To put it short, it ‘flies’ even worse than before. Unless there is another explanation, I’m really tempted to file this under hardware (IMU) failure for the winter, and replace the FC next year…
I had also checked the motors, even adjusted the tail one which was very slightly tilted, probably from all the failed attempts. About CG, I was using a 4S 1300 mAh Lipo for all the testing, not much space to move it around. But as long as it’s ‘copter mode’ it shouldn’t matter much I guess? To be honest, I have even flown nose-heavy Arduplanes and still none ever crashed.