Ok, I just did some nutty calculations - they should be alright - We can see something interesting with expose vs focus...

3 dice - focused. Expected hits= 2.25 Variance = 5.627
4 dice - exposed. Expected hits= 2 Variance = 5

4 dice - focused. Expected hits= 2.99 Variance =9.75
5 dice - exposed. Expected hits=2.5 Variance = 7.5

In addition to being able to see the extra hit with expose, There's also smaller variance when using Expose vs Focus. This means that the Standard Deviation for Expose is lower than the standard deviation* for focus. Which says we're more likely to see a value closer to the expected hit value. (Things get more convoluted when you factor in evade dice).

This also treats hits and critical hits the same - but it's pretty easy to work out the critical hit probabilities (just not so much their value over a regular hit). It's fairly obvious that Expose increases your odds of seeing more critical hits...

Expected Critical hits on 3,4 and 5 dice = 3/8ths , 4/8ths, 5/8ths... Comparing to target lock is a bit tougher - since that changes the crit probability somewhat - each die you reroll is adding another 1/8th...

*Just square root the variance for the Std deviation.

Edit: Here's a graph! Because we all like Graphs...