As described, I'd agree with the FKOB call. If the ball was going out of bounds on its own, and R did not intentionally step out of bounds, then you're going to find a lot of understanding of the call (although, K's coach may not initially agree...)
However, what would people think of a tweak to the rule that if R touching the ball causes the ball to become dead out of bounds, then it is treated the same as if R had touched the ball prior to it going out of bounds?
The net result of this is eliminating the IP foul for intentionally going out of bounds on a free kick (and our need to rule "intentional" and whether the ball would have crossed the sideline plane, etc.). If R wants to let the ball go out of bounds, then the need to let it go out of bounds. If R wants to play the ball in the field of play, then they can take the results of their actions.
Edit -- I don't have the full wording of that suggestion correct. The kick may need to be grounded first, so an airborne kick that crosses the sideline plane before being touched by R would still be a free kick out of bounds.