Has anyone preceded Shaykh Ibn 'Uthaymeen in saying that the forefinger should be moved between the two prostrations while supplicating, and what is the most preponderant opinion on this issue?

The Sunnah indicates that placing the right hand between the two prostrations is like placing it in the tashahhud, by clenching all the fingers and pointing with the index finger, or by clenching the pinky and ring finger, forming a circle with the thumb and middle finger, and pointing with the index finger. This is supported by the hadith of Ibn Umar and the hadith of Wa'il ibn Hujr, may Allah be pleased with them both.

Raising the index finger during supplication supports the hadith of Wa'il ibn Hujr: "I saw him moving it, supplicating with it." Accordingly, it is prescribed to move it with every supplicatory phrase as an indication of Allah's exaltedness. This is an additional movement to the absolute pointing mentioned in the hadith of Ibn Umar, which is done throughout the entire sitting.