Avengers 83 isn't the first appearance of a character, just the first published depiction of a character's likeness - as portrayed by an impostor. You might as well just go full Ween.
Really now, you haven't even read the issues you're discussing, have you?
In all 3 of the first appearances, the Enchantress is the one to give the Valkyrie's likeness/personality/powers to each character. In Avengers 83 she used the likeness on herself against the Avengers. In Hulk 142, she used the likeness on Samantha Pennington to attack the Hulk. In Defenders #4, she again used it on Barbara Norriss to help her with the Defenders.
What in the world is the difference?
Avengers 83:
1st time Valkyrie's name is used? Check.
First Valkyrie's likeness? Check.
Prominent cover appearance? Check.
Valkyrie's looks exactly the same as in every appearance over the next 30-40 years? Check.
Actually Joe Collector is right and here is why:
Avengers 83
( Enchantress in Valkyries form)
Hulk 126
(1st Appearance of Barbara Norriss)
Hulk 142
(Samantha Paddington in Valkyries form)
Defenders 3
(2nd Appearance of Barbara Norriss)
Defenders 4
(1st appearance of BRUNNHILDE/VALKYRIE in Barbara Norriss's body)
Defenders 109
(BRUNNHILDE/VALKYRIE in her own body