Absorb Magic is generally not worth slotting
Really depends on the content you are doing. No one I know is really using spell power and weapon power pots in base game dungeons, so for those runs the 20% buff probably out preforms the additional 1100 pen(number taken from top of my head it could be a little + or - and assuming you are running infused and perhaps torug's) you get from running a staff with blockade. For trials and dlc dungeons it's safe to assume people are using good pots so staff(if you have one) will undoubtedly out preform igneous. Most important thing to me is to run what you can sustain without relying on healer so blockade may not be the best choice for a new tank just learning the role, because if you can't sustain it and yourself and go down you have failed as a tank. It's not always the best to take a meta build for maximizing DPS while learning the tank role but gradually progressing to that stage as you learn the role.
Just my 2 cents from someone still progressing as a tank and swapping from DPS main to tank main.
And also, for passive, worth it to drop 2 heavy for 1 light + medium, or only 1, for 1 medium? How big diffrent would it give?
The difference is minuscule. Your typical tank 5/1/1 build would have ~400 more mag and stam and lower resists (the magnitude of which depends on which pieces are of different weights) than a 6/1 one, with fairly comparable sustain. I personally always go for a full heavy monster set and make only the belt and sometimes hands med/light. This way, if my mental math is right, I lose only ~600 resists and a tiny bit of mag/stam instead of ~2150 spell resist and ~2500 physical resist if I ran off-weight for both monster pieces.
Ring of Preservation? instead of Absorb Magic maybe?
Also, if I go with a frost staff, should I take Tri Focus passive? I have read that you should skip it so blocking not costing magicka, but some builds not mention it etc.
What would be best? Not that you will block so much with the staff there, but can be nice to get the taunt effect and shield pop