To me Geralt being Foltest's bodyguard is perfectly logical, not only would he be intrigued by a fellow witcher dabbling in statecraft, but he would know what a danger such an assassin can be. That's not even bringing Foltest, one of the most powerful and charismatic individuals in the north, into the reckoning. He will of course use the witcher, and ride rough shod over any raised objection. The white wolfs life is in the balance so long as he is stood on Temerian soil, and witcher neutrality will be swept aside with the morning wash.
I can't see any logical way of not having the game start out with the white wolf bound to Foltest, the attack of a witcher assassin overrides any of Geralts plans. Maybe if he had been sent on the hunt to track them down, but Foltest has more important matters of realm and state to pursue.