1.Yes the buffs stack but they have to be uniq, you cant add more than 1 defence champion as only the best would work.

2. When you add legendary you see that they need "backup" with units that belongs to them, if that is green than the percentages are the same as the units if that is yellow mean legendary as weaker and you need more troops to add there as backup or lower the number of the legendary 

3. For the last part we dont know the real mechanics behind this as developers never told us, but its better to mix the units when u defend.

i

On the same topic of hamlet champs stacking, I would like to ask, the headman and the fragment champ gatherer, does their effect stack together in decreasing time? If yes, kindly share some screenshots on the time decreased using both of them, thank you. 

I'm wondering what Pokie asks too. I like to know if it works using multiple hamlet champs. if not then headman is probably the best hamlet champ.

As much as I like to help, I also like to keep the game's mystery, the developers never gave us any information and everything we shared with you is just from our experiences as players :)

What do your teammates in your leagues think about this?

Honestly, most people don't care about mechanics. They just send units. lol

If you want to test the hamlet champs.  Secure a hamlet.  Send the hamlet gathering champ and see how she reduces your time.  Then send the Headman and see how he reduces your time.

Each one depends on how well you have built and upgraded them.

Most players will hold back their headman champ and send him when they are ready to collect.  That is how they can keep a hamlet for a long time.  They are ready to collect it and find the new one to secure.

Regarding game mechanics, Valahia is right.  Also, it is hard to give an answer because every battle is different.  The lost arts, champs, buildings, boosts, troops sent, etc are all factors in how strong the offense is that hits your defense and vice versa.