1) I go with [[Siren Stormtamer]], to enable [[Wizard's Retort]] on turn two. Even if you don't have a Retort in hand, you may draw one.

2) I try to use Spell Pierce as quickly as possible because as you said it loses value quickly. I'm probably siding most of them out against control decks tbh, since control is likely to have a lot of open mana at all times. Right now I feel Spell Pierce is strongest against [[Wilderness Reclamation]] decks and Sultai Midrange decks that like to play expensive kill spells like [[Vraska's Contempt]] and tap out for [[Vivien Reid]]. It also not bad against mono-red, 1 mana to avoid a burn spell or counter their [[Light Up The Stage]] is pretty decent

3) I'll play the creature there but hold off on the Obsession until I can protect it.