I just wanted to drop a link to my analysis of the Resto Shaman changes, buy wow account where I've covered a couple of the questions you had about the t16 4-piece bonus, etc.:
Currently the spiritual guardian summoned when you cast Spiritwalker's Grace copies your heals at 35% effectiveness, and since your initial heal was modified by your Mastery, the copied heal isn't. Or rather, it is, because it's 35% of the heal-you-cast-which-was-modified-by-Mastery, so if you had less Mastery, you'd have had a smaller heal, and thus a smaller 35%... I hope that's clear haha my brain is fried atm.
(If it were instead 35% of your base-heal-without-Mastery-factored-in and then got its own Mastery factor, that'd probably work out to less than 35% of your actual copy-triggering heal, because of course you just healed that guy so now he is on a higher health percentage! So I guess this is the higher-throughput way to do it.)
The set bonus as of right now does not interact with Ascendance or Ancestral Awakening/Ancestral Guidance. I wasn't able to spend enough time to tell if it interacts with Echo, though I suspect it won't because Echo healing isn't a cast-time spell itself. (I'll do that in enhancement spec where Chain Heal has a 69% chance to proc Echo, when I have the time, because damn, that's going to be hard to prove with only a 22% chance in 15 second window on a 2-min CD ability.)
The two-piece is underwhelming, yes, but I looked over one of my logs at random from the last month, where I killed 10 bosses, and Earth Shield was 2.1% of my healing done. Earth Shield + the t16 2-piece would have bumped that up to 4.6% of my healing done, or a 2.5% increase in throughput. That's about where I think set bonuses should be, personally (well, a little low, maybe 5% or so), but it sure is boring.
At least, being a small shield, it's likely to be fully effective healing. It won't be very exciting if your tanks are both Blood DKs though since their Blood Shield has priority over all other shielding. At least you know you'll be 2nd or 3rd in line buy world of warcraft accounts (depending on the size of Illuminated Healing shields your pally co-healers re putting up)