the walk out the back of Epcot to B&C will take you about 6-7 minutes.
seating won't depend on what you are having (meal vs. dessert), just how full it is when you happen to be there. It's a small place, not much seating to be found. HTH.
I am pretty sure there is only 1 way to head and there are signs. At one point you will see a crossroad in the path and one direction takes you to the Boardwalk area.. sort of veering off.. just keep straight and you will get there. It is really easy and quick!!
Yes, you exit through the International Gateway - there are restrooms right after you exit, just in case you can't make the walk, lol.
You stay to the right when it forks. Going left would take you to the Boardwalk.
It takes no more than 10 minutes to get to Beaches and Cream walking at a normal pace. There is a boat if your feet are tired.
As for whether or not you'll get a seat, I don't know - luck of the draw there! But it doesn't matter if you're eating a meal or just having a treat. They just seat you - they don't ask what you'll be eating.
Beach's n Cream shouldn't take much more than 5 minutes - it is very close. They make decent food too, I like the chicken soup (the burgers are good as well, though I prefer DisneyQuests).
The Beach Club Marketplace also has gelato now, for a chance of pace from ice cream.
I have walk photos around the BW, and also from MGM to Epcot - that may give an overview of where Beaches and Cream is (at least where the Beach Club is). You have to scroll down for the Boardwalk and MGM to Epcot pics, and click on the links.