The problem with multi-part lines, is that the parts not necessarily will be in a proper order. The "assumption" (here we go again) that the last part is the actual end part of the line, is not necessarily true and parts do not have to be connected or share the same direction. In this case the sample script will also apply the distance for each part and not as a total (distance from attribute divided by part count). Maybe for the sake of simplicity it would be good to assume single parts? And yes, when you apply segmentAlongLine on an illogically ordered multi-part line feature, you will obtain strange results...
