Actually, we DO know that Graendal was burned back at LEAST 15 minutes or so. There had to be enough time for Ramshalan to get Compulsed, return to Rand, be delved for Compulsion, and then for Rand to act. Given that they were a fair way from Natrin's Barrow, and that Ramshalan would take time to find his way out, I'd say an estimate of 30 minutes is reasonable. And this is the MINIMUM balefire strength - Rand could have used more for all we know, but any less and the Compulsion would not have been removed.

Based on this, we know the strength of the balefire was similar to that used on Rahvin (ie, Rand's total strength). Couple that with the area affected:



and we see that



Because we have to assume that the Balefire was equally strong at all points (Rand didn't know where in Natrin's Barrow Graendal was, and he had to hit her hard enough that he would get the confirmation of her death from the removal of Ramshalan's Compulsion).

1 million times Rand's own strength is certainly a massive boost, no matter how you look at it. Even if we assume that Natrin's Barrow is only 250m squared, that's still 62,500 times Rand's unaided strength.

So in answer to your argument, we do definitively know that the balefire was of substantial strength, and therefore covering a large area of it was a dangerous act.