The ultimate solution? There was none. The Fathers simply laid down both position and more or less pretended that they hadn't made a contradictory statement. So, Trent tells us that grace is necessary for free will to act, but then it turns around and says that free will has to actively cooperate with grace or refuse it- otherwise previent grace is ineffective.
This brings about a logical problem at the heart of the Roman Catholic attempt to balance out free will and grace. If free will acts first, and cooperates with grace, then grace is unnecessary to start the process of salvation. Consequently, free will rules the roast. If one says that grace is first necessary for free will to act, then there logically was a point at which grace was acting and free will wasn't acting. Grace had to act first and the human will had to be passively acted upon.
The problem with either of these solutions for Roman Catholics is that they want to claim both/and. We, Catholics say, affirm both free will and grace. Both are balanced out. If one isn't present and active, then the other is ineffective and salvation doesn't come about. But here's where it all breaks down. It is simply logically impossible that both act at once to begin the process of salvation. Either one starts the process of salvation or the other does. There's no balancing them out as I showed above. Consequently, they are left with an utterly incoherent position. This is the main reason why they have never stopped arguing about this issue since Trent and they probably never will.