If your 11x14 pan is 2" deep - you would not need to make any "scaling it down to fit the pan" adjustments to the recipe - it will hold slightly (6.5 cubic inches) more than 3 round 8x2 inch pans. This would be your ideal pan.
As for the recipe - I assume this
is the recipe you are loking at - follow it exactly! Baking is based on ratios of ingredients (formulas) that unless you know how to convert them - you're asking for trouble when you start trying to change them. What you have is an "oil" cake recipe - it's not the same as a "shortened" cake (made with shortening or butter). And, even switching between shortening and butter in a shortened cake recipe requires an understanding of the differences between the two and how to adjust for them (for example: 1c shortening is about 20% air, 1c butter is about 12-18% water).
Yes - you can cool the cake in the pan and then frost it ... that's how the lunch ladies at school always did it!