I mean, you could keep asking to define words in a definition indefinitely...
It's not indefinite, but rather until there's a practical aspect to be taken into account, rather than an aspirational one. (The kicker, naturally, being that you can't really attach a practical definition with universal application to a comparative state of options available to characters in a game that has little-to-no hard-and-fast situational parameters and anything can be attempted.)
Meaningful obviously sounds like a subjective element. A choice might seem meaningful to one player, but not to another. It's perhaps less important in designing a balanced system.
Viability is more straightforward. Does making the choice harm your fellow players' chances of succeeding in the cooperative game? Is it less-contributing than an equally-weighted alternative?
I disagree that viability is straightforward, here, because even if we leave aside the issues of how you quantitatively measure the chances of success (which are, at best, an issue of dice probabilities), there's even less ways of applying a metric to "less-contributing" and "equally-weighted."
The point which I'm trying to make here is that this is a rabbit hole which, similar to the prospect of some sort of immersive verisimilitude which appears to be tantalizingly close if only we add just a
few more options, can't be achieved. Or at least, can't be achieved in any objective sense; personal satisfaction is the best there is.
Chess is a very well-balanced game, but if your opponent is Gary Kasparov, then despite having an equal number of pieces of equal capability, he'll still dominate the game. That's not a perfect analogy for a cooperative game, but I'm of the opinion that the central conceit remains true: that while balance in principle of design isn't unimportant, the bulk of its application depends entirely on the particulars of any given game session, which isn't something the rulebooks can mandate.
In that regard (and to bring things back around on topic), I think that verisimilitude is an answer to the problem of balance, because it presents a rational counterpoint in explaining
why that balance isn't attainable, nor necessary to the quality of the game. From the way(s) that magic works on down through the reasons the party finds themselves in a particular scenario, the immersive quality offers a satisfactory explanation for why one character might have more options than another
right now, and that's okay.