Yes to all of your questions, unless explicitly stated otherwise.

Does the effect from multiple Norogs really stack? So a team of 4 Norogs without active buffs has 100% damage reduce? Sounds a bit odd. 

It's a decent question, and one for which I don't have the answer for. My guess is that there is a soft cap somewhere, but I don't know what it is.

it would be multiplicative not additive, so it could never reach 100%

Trips suggested to me that stacking would work in multiplicative fashion. In other words, 4 Norogs would look like this

100% of the damage x 0.75 x 0.75 x 0.75 x 0.75 = 0.3169. Which means you'd stop 68% of the damage. 

But put them all in Stalwart and AOE's now deal about 20% of the damage they would normally do. So if you put them all in stalwart + 2 immortal pieces, and add in a Vogoth, so that you could leech back your health, you have a near-indestructible PVE team....

I guess Norog is the best champion in the game to have 4 copies of LOL

In this thread on reddit they say, the same passive (same hero) doesn't stack, but different passives, like Norog's and Duchess' do. I don't have any information, if the passives stack additive or multiplicative. I don't know, where Trips got his information that it's multiplicative.

If it's additive, let's try to find a team with 100% damage reduce.

Norog (more than one buff) = 15%

Pythion +25% = 40% 

Duchess +15% from passive (only AoE), +15% from perfect veil (only AoE) = 70%.

Hellmut to have enough buffs for Pythions passive.

Venomage +15% = 85%.

All team members: Blastproof (defense tree) mastery +5% (only AoE) = 90%.

One champ Bulwark (defense tree) mastery +5% for others = 95% for others.

6th champ (Hydra only) Coffin Smasher +5% (AoE only) = 100% damage reduce for all team members exept Bulwark mastery champion. 🙀 Ok, only AoE attacks, and only if all buffs for Pythion's passive are active.