Maybe your right. But, I don't think so. I think that if that is the case, about it not being because of the Colonel, Elvis would have gone overseas in the 60's! Sure he had movie contracts, but you make the point regarding vacations as well, and Elvis never travelled that far on vacations either. The colonel was an illegal citizen, he basically scared Elvis into that whole drug situation which you are mentioning. By doing that, he managed to keep Elvis in the U.S untill his death.
Elvis bought the Lisa Marie, only because he wanted a trans-atlantic jet for a world tour. I believe he wanted to. Elvis was on medication during the early 60's.Albeit that it increased in the 70's, but he would have faced the same problem with it in the 60's, in relation to travel.