As far as using a manual Mem Depth setting as opposed to Auto... there is a complex interaction between the timebase setting, the memory depth, the sample rate, the number of channels in use, and the relationship between the total memory buffer size and the screen display area. You can explore this for yourself by setting the Display Type to "Dots" and varying the Mem Depth and horizontal scale settings, and STOPping the scope each time so you can see the Dots easily. The scope's Sample Rate and the Memory Depth it is using are always displayed at top left next to the "H xxx" timebase setting indicator. The top center wiggly bargraph thing represents the total memory buffer size, and shows the screen display area as a portion of the total buffer. Watch how this bargraph changes as you set different Memory Depths. With the scope stopped, you can use the horizontal position control to scroll left and right to see where the memory ends, for each setting of Mem Depth. (You can only change Mem Depth when the scope is running.)
For example (note carefully the wiggly bargraph display at top, see how it represents the screen area, the buffer size, and how much of the buffer is displayed in the screen area):