The way Morde was adapted into duo lane is flawed and broken designwise.
Very long post. If you don't want to read, skip to tl;dr.
Full experience for last hitting minions.
Riot... WHAT WERE YOU THINKING? Was that the best you can think of? Giving free experience to a champ when duo laning? Giving level advantage to champion for just farming correctly? And it's not a little advantage, like with Zilean's passive, no no no, it's an advantage of AT LEAST 2 levels. If Morde farms correctly, he can get level 6 while his laning opponents have level 4! Having ultimate and one of your abilities on rank 3 vs having one of your abilities on rank 2. You know what that means? Basically, laning against Morde in duo lane means waiting for unavoidable death. You can't do anything to stop Morde from getting that advantage. Poking doesn't work, because Morde has built in sustain in form of W heal and passive shield + it gets worse if he has relic shield support. Trying to kill him is suicide, especially when he has Leona or Nautilus supp, so he can easily smack you with 3rd Q while you are CC'd. Pushing lane so he looses some of the minions under turret? With that W damage that makes his minion clear insane, it's impossible. If Morde player is competent, you can't do anything, he will get that huge level advantage. This is the cheapest and one of the most unfair mechanics League has ever seen and it makes Morde impossible to balance: no matter what Riot does, he will be either OP or trash tier. This means that it is impossible to make solo lane Morde viable, because trying to buff solo lane Morde will result in buffing duo lane, making him broken again. Morde was made a duo laner because he was deemed as "unhealthy and toxic" as a solo champ. There's only one problem: his duo lane is more unhealthy and toxic than his solo lane ever was.
So, what is Riot's answer to such a state? Huge nerf to W damage. Basically, Morde lost 300 base damage from his W, because the overlapping mechanic was removed, as well as his heal was weakened. And while I don't think this is a bad nerf (the numbers on W are completely overtuned right now on live), this won't remove the cause, which is free exp. The mechanic that is supposed to make Morde a viable duo laner is the cause of him being completely broken and impossible to properly balance champion.
The W passive should be removed. Completely. It's the only way to balance duo Morde, and it's the only way to open possibilities to improve solo Morde. Riot, you want to make melee champion viable as a duo laner? Sure, why not, but it shouldn't be done by implementing such cheap and flawed mechanic like that.
Tl;dr: Morde's W passive is broken designwise and gives completely unfair advantage. The opponent cannot do anything to stop competent Morde player from getting level advantage over him. Because of that, Morde is impossible to balance: he will be either broken or trash. This also means that trying to make solo Morde viable is impossible, because duo Morde can automatically become broken again. The only way to balance duo Morde is to remove W passive, that way, he can be easily balanced both as a solo and duo laner.