Auto Scroll only works for a part of the video

Is your video one shot? Is there more than one effect applied that last the entire 15 minutes? Have you considered creating a MOGRT and rendering your long video in Premiere Pro? Premiere Pro rendering is always significantly faster than rendering in After Effects. It is very easy to create an endless scroll or roll MOGRT using this expression for the layer's position:

moveDistance = 6; //pixels per frame 
startTime = time - thisLayer.inPoint;
frameCount = startTime / thisComp.frameDuration;
pixPerFrame = frameCount * moveDistance;

	[value[0] - pixPerFrame, value[1]]

This will keep any layer moving from right to left for as long as the layer lasts. It will take a long time to run out of the maximum number because it always adds 6 to the current frame number. I think the current maximum number for Javascript is 64-Bit or 1,152,921,504,606,850,000. That is a lot of frames.

A simple Ctrl+Shift+D to split the layer will suffice in case of the presets. They have all the necessary code to honor the inpoint as the new start point.

Mylenium

Your expressions simply reach their maximum values and then the effects stop. You need to split up the relevant sections into individual segments and/or modify the expressions to reset so they start counting from zero again.

Mylenium