Is the block % and block absorption of a shield included in the calculated Protection value for that shield? If not, is there an easy way to determine which shield will provide better average damage mitigation between one with higher armor and one with better blocking?
I do not believe that the 'Protection' value on a shield when comparing to another item does not take into consideration 'Block Chance' or 'Block Value'. I say this because a shield in comparison to a magic source on my Wizard rarely leaves more than ~0.2 difference in protection, yet my shield (Lidless Wall) has a 20% block chance for 2-3k or so.
With this in mind, when comparing shields to shields, take the 'Protection' value at face value but then do your own comparison for 'Block Chance' and 'Block Value'.
You may find an EHP calculator that will do the comparison for you. For example, D3Up used to allow you to compare EHP for two items for a specific character. It will also tell you how much an increased block chance will help you under EHP Gains by Stat. Unfortunately, I don't know how he did his calculation. You could try looking it up in his github repositories: V1; V2.