The wording is ambiguous - or at least a garden path phrase. I think you want "For each player, if neither [...] into that player's graveyard [...]".

Actually, it might be even better to say "When a player is milled this way, if neither" etc. because that way you don't need all the legal targets upon casting the spell.

This is still a very complicated way to make a bounce spell.

Is this effect really worth seven lines of text over:

• "Return target non-aerial, non-aquatic creature to its owner's hand. That player mills four cards."

at ?

I want the mill as the first action and the bounce as the bonus action (or at least I think I did, as I didn't save the original wording).

I also feel I wanted the card formatted more towards multiplayer, or at least expected the card to be there, were the milling is better targeting all players and probably fine at the cost. Bounce could be okay as a bonus action here, since there's no guarantee other players won't have aquatic or aerial cards milled. seems too cheap for "Return target creature each opponent controls. Each opponent mills four cards."

• "Return target non-aerial, non-aquatic creature an opponent controls to its owner's hand. Each opponent mills four cards."

That's totally an option.

So is:

• "When you cast ~, each opponent mills four cards.
  Return target non-aerial, non-aquatic creature an opponent controls to its owner's hand."

So what's the point of the "bonus action", then? You are checking for the same criteria as with the targeting restriction and make a seven-line common.