I am remaining consistent with those cards. It only occurs below the line in existing cards and my interpretation is consistent with usage below the line.
No, "when you play this" occurs above the line in every existing card. Your interpretation is not consistent with them.
There are no existing cards where "when you play this" and "while this is in play" both apply to one effect.
Altered Highway is not a card where "when you play this" and "while this is in play" both apply to one effect. It's a card where "when you play this" applies to "while this is in play, reduce costs" and "while this is in play" applies to "reduce costs".
I am simply making the ruling that when those two wordings meet, the explicit wording wins out and destroys the implicit "when you play this". That is NOT inconsistent, it's simply a ruling to apply to that hypothetical new situation.
It is inconsistent, because it does not apply to situations that already exist, such as "when you play this" meeting "this turn".
You are choosing to make the other possible ruling, namely that the implicit wording gets prefixed onto the explicit "While this is in play". That's also a perfectly consistent viewpoint, and I agree that I have no reason to choose my interpretation over yours, they're both equally valid.
I am not making a ruling, I'm going with the existing rulings.