There was a thread a 1-2 months back about someone that finally went and did a reasonably comprehensive experiment to determine exactly what Guard does. The poster found that it was the base damage that was separated (since the two took different amounts of damage; the poster also "undid" the math and found that it only added up when you split the damage of the incoming attack into 2 separate attack "packets" with 50% of the baseline damage) and that attack type was maintained such as I explained before (the Guarding tank had 3 distinct ranges of incoming damage for the M/R attacks: more than normal for a crit, normal for a hit, and substantially less than normal for a shield). I believe that the poster also mentioned that the logs never mentioned that the redirected damage was the source of a relevant attack or not and that the only way to suss out the Guard/crit/hit was to look at the listed damage and dodges/resists had to be seen by the disparity in number of times damage was taken (essentially, Guard just says "damage taken" and doesn't even say anything when the attack is dodged by the tank).
That...is the dumbest way they could've set up those mechanics. Grar.