1. Every box you pick must follow a line back to the top row but you are free to start different paths and circle back around.
So you could end up with a talent tree selection that looks like this:
(Xs are picks)
But not something like this
Because the right tree doesn't have an anchor point on the top row.
For defense the idea behind the mechanics is that your defense dice represent your "active" defense. Things like other games would call dodge or evasion are represented in game by an increase in melee/ranged defense which adds dice. In addition there are abilities (common among NPCs) that further upgrade defense dice which again makes it harder to hit.
Your green/blue/yellow dice represent your ability to hit the target while the black/purple/red dice represent the targets ability to avoid getting hit.
While as mentioned there are a few exceptions to this the game is designed to
A) Slightly favor attackers.
B) Avoid multiple rolls for a check.
Both of those are designed for game fun more so than game balance. By making the "offensive" dice
sliiightly better than equivalent defense dice (off the top of my head, defensive dice (black/purple/red) are more likely to roll up with disadvantages while offensive dice (blue/green/yellow) are more likely to roll up with hits by like 1 face on each) you avoid problems where characters are just whiffing left and right. You're much more likely to succeed but with some disadvantages that allows the GM to allow the story progress but with a price.
The second thing is for speed of play. You'll notice that they don't have you roll damage dice, all your abilities and attacks etc. are based entirely off of that one single roll. In fact compared to other systems even contested dice rolls are pretty uncommon as even for social checks it is more typical that the GM just sets the NPCs reaction as difficulty dice on the roll. This helps the game play move as well as putting more dice rolls in the hands of the players which is a nice relief for the GM.