Michael_x, thank you, I answer some of questions in one post and first time possible I do a video so you and everybody kindly helping me here can see it in the practice and provide information about P and its variations.
Current state of project:
A) holding robot in air with hand, not putting on the ground:
There indeed is a +/-5deg with relation to vertical state, where leaning the robot won't start motors, if I lean some 2deg, motors won't start, but a "sound of noise" only goes sharper and sharper until they start. If I add to P or D, or both, the reaction happens faster and with even 1deg, motors react correctly, but then they also remain wildly active: just little bit of movement and they stay rotate too fast.
B) putting on the ground:
Same behavior nearly, but now that +/-5deg becomes +/-15deg,
Now, if I add any "I" term, robot becomes totally crazy!
I can't say that sensors are giving wrong data or giving it right but late: if I connect the bot via serial to PC and monitor output of filter, angle changes very realistic: on 0 it stays on, like, 0.40 and all the time oscillates like 0.34, 0.26, 0.66, ...but it is on 0 stable! then a 1 deg. movement outputs 1deg in same form: 1.33, 1.53, 1.37, 1.22,...
All the problem I see is in parameters of PID, and as I've been pointed to, the chassis construction for what I have to add to momentum.
Also, I can't say that zero is calibrated correctly: I have no way to set it exactly where I feel the system mechanically has equilibrium. I turn it off and guess in what position it is so close to equilibrium, then if I turn on by connecting Arduino to PC and reprogram to adjust the A_ZERO (zero of accelerometer) to have it know the more correct zero position, then when I disconnect from PC and connect battery source, I get a different zero! It changes based on what power is powering.
I have no encoder available, I can make optical encoders for motors but it takes time. I think I will first try to change construction to add to momentum as described for me here, to see some more balance, then go for encoders.
Also, Please let me know, if it really helps I can reconstruct it in 2m height. That is no problem and if you see it pays back in easier balance, I'd do it.
I want to balance it 