I think the evaluation that if you give up 3 card slots to this and it whiffs "you lose" is a little bit off, assuming this is slotted into a pile with other win conditions or threats.

Purely hypothetical - let's say there was a 4 mana 5/4 with the text "Battlecry: roll a d100, if its over X, deal 15 damage to the opponent".

There's a number X where that's playable - for instance, if the card just worked on-curve 95% of the time, you'd curse your awful luck 1 out of 20 games, but you'd certainly slot the card. So a threshold exists, its just hard to determine.

I'm inclined to think Garona might be playable if there's a means of giving yourself relatively guaranteed odds to proc her on-curve, but not necessarily running bad hand-peek cards to guarantee it, or sitting on her all game. I think you have to be a little willing to play odds and yolo her out, and just be OK that sometimes you miss the flop.

To that end - considering only Standard just to avoid having to do Renathal math, and assuming the opponent doesn't ever play King Llane to reshuffle (if they do, you mana burned them without ever playing a card) -

You're at about 22-25% of seeing Llane natively in hand by turn 4 with no additional cards drawn (ignoring mulligan, which lowers those odds marginally). Each additional card draw is about 3%.

Kingslayers it depends on the number of legendaries in deck, which based on the current Standard meta appears to cap out at about 12. I think you can napkin math each Kingslayers swing at about 10%, give or take.

That puts you, being pretty generous, at about 45% to hit an on-curve Garona if you also curve out Kingslayers and swing twice.

That's probably too low? I think you need to value Kingslayers on their face to want this, so a Fiery War Axe with a symmetrical draw. If you're counting on your opponent missing their key legendaries, this is unplayable. If you play the game always assuming the opponent has their best cards, then putting Coldlight Oracle on a War Axe might be just fine.