The block chance is the size of the die on your base skill roll. So a shield is 1-20+all of whatever, and a buckler is 1-10+all of whatever.
The block power is the effect of the shield as a piece of armor on your arm/hand, in case your arm/hand is struck, and it works the same as any other armor. The upstep is from the grasp.
Shield quality effects the block power as with armor, and it also effects your block rolls as in a weapon attack roll (using your shield skill).
It'll do a simple die skill roll against your armor skill (as a flat number without any randomization, though all of the detrimental effects apply). You don't get a huge trance bonus here, but it does prevent pain/exertion from lowering your effective skill. Depending on how well you do, you get a x2, x3 or x4 multiplier to the damage blocking. It's pretty hard to make these rolls though. The x2 comparison is beating 0-49, the x3 is 0-99, the x4 is 0-999. So a legendary will quite often get the x2 bonus, but rarely the x4. You try the rolls in turn from x4 to x3 to x2, and you stop at the first one you get. Altogether, kind of a quirky little thing that was thrown in.
Your projectile activates as hitting grounded opponents once it flies past your intended target/target tile. It can hit intervening standing targets -- it doesn't look like it respects your intended target at all in this. You cannot hit your buddies (unless you are tantruming).
The crit formulas are highly variable and numerous. In general, you attach a percentage to the attack given by MIN((hitroll/8 + damage)*2 + agility*5 + 3 , 50). Then you roll that number against 100 for various effects one by one. If you have a crit boost on your weapon type, you can reattempt your rolls that many times for certain criticals (general those that involve sticking deep, like a circulatory part shot). Sometimes a critical also has a damage condition, like the damage from the attack has to be at least 5, in addition to making the roll. Most of the criticals also have damage type restrictions, such as the inability of burning damage to knock you out on impact.