Skeletons have gotten better in the last few leagues, certainly, so I see where @fdsaion is coming from. Using skeletons requires some investment, though - at least a couple of support gem slots if you want decent damage from them, at least one jewel slot (and usually more), and you have to take time to cast them on a regular basis. Zombies require no supports (though I usually give them Fortify, shared with my golem, to minimize resummoning) and no jewel slots, and you don’t have to do anything in battle for them other than occasionally Convocation when they get stuck on the wrong side of some terrain. In either case, skeletons or zombies, fitting in a jewel with minion chance to taunt on hit will make them much better tanks.
I wouldn’t go Blood Magic. Your mana costs as a summoner are dirt cheap, since the biggest cost stuff is summoning zombies/spectres and that’s out of combat anyhow. You’re better off either keeping the auras, or using MoM for the eHP. Purity of Elements is nice for gearing, as you mentioned, and it’s also helpful in keeping the spectres from croaking when you get to heavy elemental damage bosses (Hydra and Shaper come to mind). Even better if you took Commander of Darkness.
I’ve never bothered switching in Slower Projectiles, but it would make sense against tough bosses where adds aren’t a big deal. I’m too lazy to gem swap. :) And I took down the Shaper with no skeletons on Niekramant last league, so it’s certainly possible. Though I should point out that was with Vanguards, not Solar Guards.
As for zombie damage, they’re not gonna do much against bosses, and even with no offensive support gems they can clear trash mobs. So there’s no need to really consider zombie damage in your aura/jewel/gem planning. All you really want from them in boss fights is to provide extra targets so the boss is less likely to hit the spectres.
Never tried it, but for clear speed I can see how Pierce would be a winner. I’ve never used it largely because clear speed hasn’t been an issue when using Solar Guards, so Pierce always felt like overkill while Elemental Focus would speed up taking down rares and uniques. But hey, if it’s making a difference, more power to ya!
You’re going to have a tough time capping block if you skip Mistress of Sacrifice (and use Bone Offering instead of Flesh Offering). Three is probably the ideal number of Reckless Defense, for 12% block (assuming you can get 4% jewels).
If your primary damage source is spectres, absolutely take EE. Makes a huge difference when all that damage is fire-based. For skeletons, it’s still pretty good if you’re running sources of elemental-on-top-of-physical (Hatred, Grip of the Council). I used it on both Charming Ossifier (skeletons) and Niekramant (spectres). You could drop it for a pure physical skeleton build, but it’s only one skill point on the way to Gravepact/Shaper/Constitution so it seems like a no-brainer if there’s any elemental damage on your build at all.