Bone Skewer

Yes, as the untargetability is inactive in hand and hand cards cannot be targeted in any case.

Ruin Runner and Baron Nashor are made untargetable by passive abilities declaring that they can't be chosen by some category of spells and abilities.^[757][757.1] Passive abilities of permanents are typically active only while on the board.^[365.1] Abilities that function in another zone self-describe that context,^[366.1] which these untargetability abilities do not. They therefore have no effect while the unit is in hand. Beyond that, Bone Skewer never targets the chosen unit in the first place — cards in hand are never targets because the hand is not a public zone,^[355.10.a] so there is no finalization step at which a unit there could be declared as a target.^[355.5]

Yes, because the reflexive trigger that stuns the unit does not ask its controller to choose anything.

If the opponent plays the unit, Bone Skewer's "then do this" wording creates a reflexive trigger as a pending item on the chain.^{[387.1][387.2][388.1]} The trigger is linked to the preceding ability, and "it" can only refer to the unit played by that ability.^[394.1][397] When the trigger is finalized, no choice is made about which unit will be stunned; it is always and only that unit. Because the unit is programmatically determined rather than chosen, it is not a target.^{[355.10.d][355.10.d.1]}

This means untargetability — the "can't be chosen by enemy spells and abilities" passive on units like Ruin Runner and Baron Nashor — does not prevent the stun, since it only stops a unit from being chosen as a target and the stun targets nothing.^[758] Deflect is similarly irrelevant, since Deflect is only added as a cost when a spell or ability chooses an opponent's permanent,^[809.1.d] which is not the case here.

No, hiding the unit does not fulfill Bone Skewer's instruction to play it.

Hide is a separate action, not a subset of play.^[811.1.c.1] Even though the chosen unit is in hand and may have Hidden, Bone Skewer instructs the opponent to play it as a limited action.^[419.3] That play follows the normal process unless Bone Skewer says otherwise,^[419.3.b] so the unit is put on the chain and queued for finalization.^[419.1] The opponent cannot choose to hide it instead.

Yes, as declaring the intent to pay is what satisfies the condition, not the amount actually spent.

The phrase "any and all costs" in Bone Skewer's text is intentionally broader than the standard "ignoring its cost" wording. Standard cost-ignoring only zeroes out a card's base energy and power costs,^[356.1.b.1] whereas "any and all costs" sets the card's total cost to zero, including non-standard costs like additional costs.^[356.5.a] Despite this, the opponent may still declare their intention to pay an optional additional cost during step 2 of playing the card — the decision to pay is made before costs are determined. Once declared, that additional cost is folded into the total cost, which "any and all costs" then sets to zero, so it's already 0 when it comes time to pay.^[356.5.a] An optional additional cost is considered paid based on the decision to pay, not the amount actually spent,^[356.4.f.1] so the condition is fully satisfied. Any conditional effect tied to "if you paid the additional cost" therefore triggers normally.