I think from Mostar to Korcula in one day may be overly aggressive, unless you were going by car (that would have its own problems with the return, so I wouldn't do it). I am a big fan of Mostar, so I hope you don't skip it. It has a very different feel than Croatia. In the Balkans, you just have to go with the flow especially when you're at the mercy of a public transport schedule. I ended up going via train/bus combination and was unexpectedly transferred from a train and then onto a bus in a section where they were doing some reconstruction on the train track. My luggage ended up on a totally different bus that I was on; yet, everything worked out just fine in the end. I had no choice but to trust the folks over there, and luckily, they were very honest.