Well he doesn't really need the MR in the jungle so much because he is in the jungle, I tried it like you and found it is better to give him flat AP Glyphs (blues) because it helps increase his W's heal.
Personally, I feel that Stalker's Blade works best. The reason is that Mordekasier DOESN'T need items as much for help in the jungle for clearing, his kit makes him perfect for this. But I can already see two issues with him: ganks, and counter jungling.
Yes, he will be at high health throughout the jungle, but people could just come in, steal your camps right when you are there, and just walk away from you. That has been his issue in lane, and it is his issue in the jungle as well. Counter jungling usually works by junglers coming to steal your camps, or trying to kill you, and while any jungler would be insane to try to kill you while you are jungling, they can very easily take what you have and not have to worry about dying, since only arrogant players would try to do that against a Mordekaiser if they were at low health and had no escape outside of flash.
The ganks are the other issue, it is annoying enough to try to gank as Mordekaiser because he can literally only stomp into lane at a slow as hell pace, he would have to rely on the laner to cc as he comes in, using his W to get in, and then what? They'd likely just escape and Mordekaiser has nothing built in to catch someone. He NEEDS Chilling smite to slow others and speed himself up, especially if he wants to land the three Q's of death.
Ranger's Trailblazer and Poacher's Knife are only useful if you want to be clearing camps very quickly, if you intend to be a fast jungler leveler who backs rarely, then I guess they can work. Skirmisher's Sabre is more if you intend to duel people, which you might considering it works well with the bruisery Mordekaiser who uses bulkiness, shield and damage to out-duel a lot of people. But you take Skirmisher's Sabre knowing you won't be able to make killing ganks unless the laners never ward, you're lucky, or at least one ally laner can do some serious cc-chaining like Leona in bot lane.
As for the upgrades...
Devourer is only good if you build an AD/AS-focused Mordekaiser, since he doesn't benefit from phantom hit in any way. Same thing with Warrior.
Runeglaive is... marginally effective on him, the mana effects are completely wasted on him... is anyone here thinking anything that gives mana to manaless champions should just give like... health but at a reduced value, like say 50% value instead? It's useful if you want the spellblade effect since it gives you more reason to build Sterak's Gage on him to buff his Q, and you can build another item in place of Trinity Force... maybe you can build Boots of Swiftness for movement and then another item slot for whatever you want.
Cinderhulk is useful if you want to go tanky Mordekaiser, the health bonus works wonders with Sterak's Gage, again if you build it.
