THE GAME DOESN'T END AT LANING PHASE.
I know. I made my argument knowing that. It is negligible when he's solo laning, even with this taken into account.
But saying that because he doesn't use it in the laning phase it isn't part of his power budget
Correction: He doesn't use it during the laning phase, and it's incredibly weak outside of the laning phase.
Being able to get full exp for any creeps you last hit when your ENTIRE ALLIED TEAM IS THERE is pretty powerful,
I'm going to disagree. The vast majority of solo Morde's CS throughout the game is going to come without any allies being around, and by the time of the game where this does start to come into play even a little, xp differences that minor become extremely weak.
It is as nonfunctional to a power budget as any mechanic in the game. Even just saying that this gives him power is overvaluing it by an ungodly degree.
Dragon Ghost is harder to utilize as top lane Morde than your W passive - does that mean it's not part of his power budget too?
No, because it still has some potential strength that he can tap, even if he does so rarely. The XP passive doesn't.
Any buffs or nerfs to Morde that didn't take this account would just catapult him back into bullshit bottom-lane territory, and that's why you have to consider it when buffing/nerfing Morde,
That's an entirely new argument. It's also one I happen to agree with. Morde's potential to go bot lane does mean that when he gets buffed, certain buffs aren't options because they would support bot lane more than solo lane. That is a design limitation.
Now, that doesn't mean that the XP passive is currently part of his power budget. Instead, what it means is that it has the potential to be part of his power budget, and we have to impose design constraints to avoid this. That is a good argument for removing it, and I suspect Riot will do so at some point in the future for this reason, particularly if they ever rework Morde.
It also doesn't mean that his power is limited as a result of this design limitation. We have already proved that we can buff Morde while helping his solo lane more than the bot lane. That means that we can continue to ignore the XP passive, so long as we are comfortable with limiting his design options as a result. (I'm not saying we should be comfortable with limiting his design: just that we can easily alter his power at will without risk of bot-lane Morde so long as we do).
Like seriously, look at for an example of a champion's complexity and small mechanics pushing them into OP land.
I wouldn't say that it is small mechanics that are making Azir unbalancable. It's his entire kit. His problem is that by design, he has an answer to every situation, so long as you and your teammates can afford to play very passively during the stage where you are weak.
Having an answer to every situation is not a complaint anyone could ever lodge against Morde.
The same thing could easily happen to put Morde bottom lane again, as long as the W passive exists, and so it's not "unrelated" to his power level in any way.
This assumes that we can't buff Morde without risking his bot lane becoming a problem again. That's just not true. We've already proved that we can alter the relative strength of Morde's roles through selective buffs. We can continue to do so easily in the future. That means we can buff him as we choose without risking bot-lane Morde. That's how we got away with reintroducing solo-lane Morde in the first place.
Again, the problem with the XP passive is that it imposes design constraints, not that it imposes power constraints. We can easily give him whatever power level we want without bot-lane Morde, but we have to avoid certain buffs to do it. That's not a good thing, but it's a different problem than the one you're talking about.