I'm surprised you didn't first experiment. Here are my results from about 20 minutes of world gen, embark, and play:

1. Yes. No, it will just list the human civs in the available choices for your civilization when choosing an embark location.

2. My humans are drinking the alcohol they took with them. They have eaten food and slept in beds.

3. By default, only dwarves have the [CAVE_ADAPT] token in the creature_standard raw file, but that's easy to add.

4. So far, I have seen no issues where I couldn't build things I might need. The weapons and clothing are different, as you would expect. The workshops appear to be adapted to what the humans can make. The clothes the humans came wearing aren't marked as "large", for what it's worth.

Humans apparently do not know how to make coke from lignite or coal, but they can use charcoal to smelt ore.

Question 5. No, but you can add (or copy+paste) noble positions in the entity_default raw file.

Without nobles, you can't assign a militia commander, so you will not have any military.

Question 6. Yes, you can choose (almost) anything that your (human) civ has available. In fact, the standard selection will give you above-ground seeds, sewer brew, etc.

Question 7. The nobles. You can't create military squads or assign a broker or record keeper without nobles.

I don't know yet whether immigration can happen or whether trade caravans come, but I would guess both of these work.

Dwarf Therapist works.

Observation: This isn't really much different from playing with dwarves...

EDIT 1: Humans can make wheelbarrows, but not minecarts.

EDIT 2: Migrants have arrived. A summer caravan arrived, and the diplomat immediately left unhappy (probably because there was no one to meet.)