I'm not sure you're right about guard mechanics. (Actually, it's more than I hope you're not right and suspect you are.)
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).
I'd understood that when guard is active it adds a 5% damage reduction to the affected player
There's something that a lot of people don't realize about Guard and that the 5% DR isn't additive DR (meaning it's added to an existing number like your K/E and I/E DR) and is instead mutliplicative DR (meaning that you simply multiply what you would have taken if Guard were not present by .95 in order to determine how much you actually take). As such, the 5% is really *really* hard to actually notice. 5% additive is a *lot*; 5% multiplicative is... somewhat less so.
I think you're wrong that a properly played sage is less able to disengage, although I definitely agree that the commando is lacking there.
It helps that Scoundrels have a hard disengage in the form of Disappearing Act, which Sages do not have. Sages have excellent mobility (they've still got the worst mobile healing imo, thanks to the bubble lockout) and control, but it's not the same as the ability to disengage: after you use Force Speed or Force Wave, melee characters can still use their gap closers on you (I always laugh when a Sorc attempts to gank me in world PvP, proceeds to get his face trod in, and then attempts to flee only to get locked down by my using Force Speed, Force Pull, stuns, etc.) so it's not as if you're really able to reliably guarantee a successful disengagement. If you have someone else playing interference, you can often manage to get away with it, but most people can do so if they have a buddy playing interference regardless of class.