First of all I want to know the easiest and most convenient way to cross the Laos-Cambodia border and second, seeing that the Preah Vihear temple is near after the border crossing in northern cambodia, why not head there before going to Sieam Reap. It is possible?
Yep, It is. I did it going from Pakse to Ubon, from there to Sisaket (a beautiful place btw) where I spent some days exploring the surroundings, and from the bus station in Sisaket I took, early in the morning, the songthaew to Khukan, from there I took another songthaew to the border at Chong Sa Ngam, I crossed the border and, finally, I took a moto-taxi to Anlong Veng and a bus to Sra Em, very close to Preah Vihear temple. All in all, from Sisaket bus station to Sra Em, It lasted around 10-11 hours and I think It was really worth the effort because Preah Vihear is absolutely stunning mainly due to It´s stunning location. I had heard that the border between Cambo and Laos was full of tourist and, as I was looking for something off the beaten path, I decided to try this route. It was a wonderful experience, easier than It can appear as you read this right now, very straightforward. You can get info about people travelling from SR to Preah Vihear and back even in a day trip, so that can be another option once you reach SR.
Ok I see, your route sounds fun. But what if I decide to go further south in Laos in Muang Khong because I want to go see 4 thousand islands as well and I have already been to Ubon and Sisaket?