No I never played that Mordor game, but the "paradigm" is very common. Even in Baldur's gate or other AD&D, the usual party was 1-2 fighters, 1 thief, 1 healer, 1 wizard.
I don't understand why you want to have 2 dwarf healers?
jokes apart, there's a reason: in my game you could want to have 2 healers (both dwarf or different) because you can then heal 2 tanks (a warrior and a paladin maybe). Simply as that, each healer heals one tank.
About the questions:
1) well hard to answer now, but I can say NO. because there won't be any "unlockable key that requires a thief". If you have a thief you can pick lock, but if you have a strong warrior you can bash it open, or in any case you'll always find a key
2) with skills, yes. Both for casters and fighters. Example, 2 identic human thieves (halflings aren't present in my game). You can specialize each one already with weapons: make one use always piercing, and another throwing. You'll have one deadly from close combat, and another from distance. Same for 2 ice-elf wizard: if you make one cast only red magic, and another only yellow magic, already in the middle of the game after some levels you'll have 2 different wizards.
3) no, so in theory your reasoning could be right - but since a druid would have much slower White Magic increase, he couldn't cast some powerful healing spells.
To explain better, each spell will have a requirement on each of the 6 "color" of magic. So we could have 1 druid and 1 healer both level 5:
Druid with white magic at 30 skill
Healer with white magic at 50 skill
A spell called "Healing Breeze" that has min. requirement of 50 white magic skill: only the healer could use that spell. Instead the druid could use variety of lower level ones of different kind.
Also consider that all races have 2 deities, each one with his own set of spells.
Right now is a bit confusing I know - but once I make some tests I think I'll be able to show you some screenshots or better clarifications