That does not mean they are nomadic.
His mission was to alert the Titans if it needed purging, not to purge it himself.
That is not 'following'. It is not the demon's choice.
Illidan was able to because Sargeras wasn't around. If this is a discussion about Sargeras versus the Old Gods, the Old Gods would have to show themselves a better leader than Sargeras. Considering Illidan proved his worth by defeating the commander of the demons who would later serve them, I don't see how they could sway them from Sargeras.