Hey there! Great questions, and agreed that it's always lovely to know design intentions behind games you enjoy -- it helps me facilitate my sessions as well, as a game master.
For Wield a Power, the 3 strain is a copyediting error, a vestige pre-playtests where it was discovered that 3 is rather punishing and often triggers the Suffer your fifth strain move. So if the player would like to achieve the effect anyway, they'd only have to suffer 2 strain. This would be in addition to marking 1 strain if they're "desperate for this to work"; i.e., you pay the price for 1 strain before making the move, and you can choose to suffer an additional 2 strain if the result is not what you wanted.
For Challenge Someone Dangerous, the motivation was to hit on the themes of Divine Moves being dangerous, obvious, and intense. I wanted to tap into the spectacle of divine conflict. The Mortal Moves about negotiating your relationship with other people (connect with someone, feel someone/something out, etc) are rooted in this intimate, vulnerable tone; the idea is that when you're a mortal, you can resolve things privately, with both of your hearts laid bare. That's less prevalent when you're tapping into godhood. When you challenge someone dangerous, you're not just challenging their abilities, powers, or wits; you're challenging their reputation, what they stand for, their very domain itself. Hence, requiring an audience. For what it's worth (and this is something I'd likely expand upon in the full game!), the audience can be defined however you like, whether it's literal (e.g., a herd of that god's mortal worshipers) or more metaphysical (the eyes of destiny are watching you in this moment). This adjudication has led to some really interesting uses of that move in my own private games and at my playtesting tables!
If there's really no way to involve an audience in triggering that Move, and this is proving to be a point of friction at your table, you are always free to adjudicate the outcome however you like. I'd lean in the direction of "no" to "the audience is on your side" and trigger the Divine Move anyway, just because its resolution can be super juicy to play through. This is all good feedback as I approach finishing my words on the full game.
Finally, re: keeping secrets, it's what I've found to be particularly fruitful at my own tables running GODKILLER as a duet experience! Most other PbtA games involve more than 2 folks bouncing back and forth with ideas, and I discovered quite early on in my playtesting process that if my partner knew everything I did, and/or had complete equal power over what would happen next, it took away a lot of the spontaneous fun of playing for them. They still wanted to discover my plans, thwart my plots, and interrogate my gods with various hidden agendas. This approach does put a larger creative load on the GM, which is something I'm considering deeply as I tackle the full text.
Thanks for the feedback and I hope you've been enjoying playing the game so far! <33