MaxQuest

Well... it's from patch 1.2.0 (from 29 june 2018):
 

So someone with 3 MIG, INT, PER is as effective as someone with 20 MIG, INT, PER at using scrolls and potions...

Dex increases your action speed.
While the tooltip displays action time decrease.
 
1 - 1 / 1.30 = 0.23

Yes. But you can turn it off by disabling cheats, if it's uncomfortable.
 
P.S. Btw, Q does exactly 1/4 max hp damage. So the mouseovered target will go from Unharmed->Healthy->Hurt->Bloodied->Dead.

Well... 
In v3.00 base focus gain (per point of damage dealt) was increased from 0.25 to 0.35.
 
0.35 * 1.33 (draining whip) = 46.55%
0.35 * 1.33 (draining whip) * 1.25 (talisman) = 58.18%
 
So you get 58.88 focus per 100 (post-DR) damage dealt (while being above 50% endurance).
 

How have you arrived to 83.33?

Nothing strange here
 
Unenchanted scale armor gives:
7 general DR
9 (i.e. 125%) vs slash and freeze
5 (i.e. 75%) vs pierce
 
Unenchanted scale armor with +3 vs Pierce gives:
7 general DR
9 (i.e. 125%) vs slash and freeze
8 (i.e. 75% from 10) vs pierce
 
Fine scale armor gives:
9 general DR
11 (i.e. 125%) vs slash and freeze
7 (i.e. 75% from 9) vs pierce
 
Fine scale armor with +3 vs Pierce gives:
9 general DR
11 (i.e. 125%) vs slash and freeze
9 (i.e. 75% from 12) vs pierce
 
That's why you see pierce DR combined with others.
 
P.S. Get a plate armor, and enchant it vs pierce as it also provides a bonus vs it

You need to go to Stalwart. To the left of ex-burned house, there will be a dead assassin. The dialog with an NPC near that corpse, will unlock the Mowrghek Ien. You can check here for extra details.