Regarding the crit cap, I don't believe this to be the case
I've been running icc almost everytime with an arcane mage, I have 22% (ish) crit chance, add focus magic for 25%
On boss fights I've been doing my rotation for 5 stacks of shadow weaving and waiting just a wee bit longer for arcane empowerment, then procc inner focus with nevermelting ice crystal to boost crit by 45% then applying swp
As far as I can tell the crit is the 'intended' 70% rate as the mage gets the focus magic benefit almost all the time, but atm we haven't had enough tests.