Why is it that so many people call the Nerevarine a Demi-god if Indoril was never a god like Vivec, Almalexia etc; he never took power from the heart. Is it safe to say that the Nerevarine was just chosen by Azura to follow in Indoril's footsteps?

Demi-God more so in power than literally. He's got some ridiculous tools at his disposal and he's also immune to aging and disease.

He can still die if physically wounded but is probably strong enough to fend anybody off.. which makes him sort of a demi-god. Kind of like if the Last Dragonborn became a Vampire Lord.

So technically, he actually became a demigod following the footsteps of a warrior who was betrayed by a group of demigods..

Yup. He's strong enough to beat a (3) god(s) or at least an omnipotent being(s). He fits the bill.