Questions about the gameplay of Heroes 6.

No. These are just difficulty levels, where the initial resources and troops differ.
Then, in Heroes 6, what is the "AI" difficulty level? If, according to you, the difficulty levels in Heroes 6 are "easy," "normal," and "hard," then these are levels of artificial intelligence, right?:) No offense.

It's not just the initial conditions that change, but also the AI's behavior. This is a common principle for adjusting difficulty in the Heroes series.

What are the "Tears and Blood" paths, and can both paths be upgraded?

Each path provides two perks (active or passive abilities). You can only upgrade one path. Only the main hero can upgrade paths.

Are skill points only awarded upon reaching a certain level, or can they be earned in some other way?

Each level-up grants one skill point. This has always been the case; nothing new.
Now, you simply choose not from a pre-selected list, but from the entire list of available skills. The only thing is that they are divided into three categories based on usefulness (some are taken from the first level, others from the 5th, and the last from the 15th).