Can I have 2 planeswalkers out at once?

All planeswalkers are also legendary. This means you can't have control of two planeswalkers with the same full name at the same time. You can keep both planeswalkers as they have different full names.

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One may also ask can artifacts be tapped the turn they are played?

Yes, as long as it's not a creature. Only creatures are affected by summoning sickness. If March of the Machines was in play or Ratchet Bomb became a creature by other means, you wouldn't be able to tap it if it came under your control that turn. Otherwise, it can tap immediately. Thereof, can deathtouch destroy indestructible? Indestructible creatures also ignore deathtouch. Normally, a creature is destroyed if it takes damage from a creature with deathtouch. But since indestructible creatures can't be destroyed, they're immune.

Does the rack target MTG?

The rack never targets anything, there is no interaction. Subsequently, does hexproof stop deathtouch? No, Hexproof only means that the creature can't be the target of spells or abilities your opponent controls. But Deathtouch is not an ability that requires targeting. Deathtouch is a static ability that means any amount of damage dealt by a source with deathtouch is enough to kill the creature receiving the damage.

Then, do board wipes affect shroud?

Board wipes can kill shroud creatures. Board wipes don't target creatures but rather destroy or otherwise remove all creatures at once without targeting any of them. Can you target your own creature that has shroud? As we learned previously, a creature with hexproof cannot be the target of a spell or ability an opponent controls. A creature with shroud cannot be the target of any spell or ability - including yours!

Subsequently, does lightning greaves protect against board wipes?

Lightning Greaves is your best protection for spot removal but for board wipes, you have to rely on indestructible effects either from totem armor auras or from equipments (Darksteel Plate) or spells (Ajani's Presence, Rootborn Defenses, etc.)..