I think we have to be careful to overestimate the importance of foreign intervention in the decision process leading to the EP. Certainly, removing the risk of foreign intervention was a bonus, but the EP was still a radical move.
I think the more important factor was the fact that Lincoln realised that winning the war, but keeping slavery would not solve anything in the long term. Slavery had become one of the most overriding factor in federal politics in the past 50 years. Southern and Northern politicians were always trying to make sure that "Free" and "Slave" states were at parity. You also see that the issue was becoming more and more corrosive, a lot worse than what you see now in US politics. I think Lincoln realised that preserving the Union, but keeping slavery, like had been done in the 1787 Constitution, would just paper over the problem and garantee another war in another 50 years.
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