False on the crit one. Normal crits are about 700, yes, BUT Thornmail does it based on the damage the attack WOULD HAVE done prior to armor. If they have 300 AD, you have 200 armor. About 100 from normal stuff plus 100 from the thornmail, usual. Old Thornmail would do 90 or it would do 225 on crit (2.5x from IE), but you'd only take 33% of the 750 crit unless they had LW. New Thornmail, you'd do 45+50=95. On crit, it'd do 112.5+50, which is 162.5. On crits, it's noticeable lower, but that's only on crits. This makes normal ADC builds more potent while Vayne, Yi, and other On-Hit based builds considerably weaker to Thornmail since they won't be critting enough.
Theoretically, you only have 55% crit with PD and IE. Say you have 300 AD. In 4 attacks, 2 of those will likely crit, 2 of them won't. That means it'll do 190 on the two non-crits and 325 in total on the crits, totaling 515. Old would've done 180+450, totaling 530. A difference of 15 damage in total. If you have more armor items, you can easily make up the difference since to make up for 15 damage, you only need 60 armor extra to total the 15 (Using the basic math formula for .25 times X equals 15, so you'd divide 15 by .25, totaling 60). Even high crit builds will be taking significantly more damage from the scary armor tanks like Rammus, Malphite, and Taric.
I like numbers. :3