I do believe that British and/or French intervention would have saved the Union, under the proposed conditions or any others. First of all, at the begining of the war slavery was not the driving force behind Confederate revolution and the Unionist response (infact, when one Union general attempted to make slavery an important point and declared abolition of slavery in Mossouri, Abraham Lincoln had said general removed and the declaration of abolition nullified). However, Abraham Lincoln later changed his mind and with the Emancipation Proclamation made slavery the central focus. While it would be nice to think that Lincoln did this for moral reasons, the real reason was that the North at this point was not doing very well and had a very low moral. By shifting the focus to slavery, rather than the vague notion of states' rights, he united the North in a moral crusade against the heathen practices of the Old South, giving the Union the moral boost, and volunteers, it needed to win the war.
Now, the Emancipation Proclamation came after the Trent Affair. If the Confederacy had declared abolition in exchange for British and French support, then slavery would not have been an option for Abraham Lincoln in uniting the Union on a moral crusade against the Old South. This would just add on to the fact that British and French support would give the South more than enough power to take on the North. It must also be remembered that the South had more than just General Lee. They had much better military leadership on all fronts, they also had a much larger amount of troops at the begining of the war (IIRC something like 2/3-3/4 of the U.S. army at the begining of the war was made of Southerners), and last but far from least, the South was fighting for something they believed in and understood. The Confederate soldiers were fighting for the homeland, they were fighting for, in their minds, liberty, justice, and the pursuit of happiness. The Unionists, on the other hand, were fighting to keep a chunk of land most had never seen under their flag, not quite the same emotion builder. The last reason for the Confederacy coming out on top from such a situation is the France factor. While it is true that the British military and some of the navy could possibly have been distracted in Canada, the French would have not been so distracted and would have been easily able to concentrate their forces on the Northern homeland. Furthermore, while British military and some naval power could possibly be distracted in Canada, that does not mean that their entire navy, several times larger and more powerful at this point than the Union's, and their industry would be. It is my humble opinion that even with a Union attempt at distraction in Canada and any other number of small setbacks, the combined power of Britain and France along with, maybe even more importantly, the lack of a point for the Union to gather round, the Confederacy in this situation would have surged back to attain their independence, and possibly even more.