Re: International Gateway
It'll take you a similar amount of time to reach World Showcase from Epcot's front door, or to take the boat from Studios to International Gateway (at times when neither park is closing). On a day when you're arriving at WDW, there may be too much adreneline in your systems to sit patiently as the boat makes three stops along the way before it arrives at Epcot, so maybe walking from the front of the park will be more satisfying. On the other hand, if you're feeling exhausted, maybe a slow boat ride is just what you need.
The nice part is that CSR is so close to both Studios and Epcot by bus. At least that part of the trip will be bearable.
Driving to BoardWalk will be fastest all around. As a CSR guest, BW security shouldn't put any restrictions on your ability to park. The walk from BW to International Gateway is pretty short, and you won't have to worry about the wait for a boat or bus at park closing (it can be quite long).
And heck, even a cab ride between CSR and BoardWalk will be pretty cheap, if you're in no mood to drive.