Yogi: Could investigating doubt be right meditation? 

Sayadaw: You can. All 5 Hindrances (Sensual desire, Ill-will, Sloth-and-torpor, Restlessness-and-worry, and Doubt) are also just objects in meditation to observe and understand. When you understand them, they are no longer hindrances to the practice.

The nature of the Hindrances is to hinder, and when you understand its nature – when you can take its nature as an object and observe it and learn about it, you understand enough that it does not hinder you – the poison then becomes the medicine.

This is Dhammanupassana – when it is Dhammanupassana, everything is dhamma; whether wholesome or unwholesome, they are just objects.

They are nature, and you just observe them as such (i.e. different minds doing different jobs).