My sleeves don't fit on the free arm on any of my machines (even Becky). When things do fit round it, they inevitably emerge wonky with pins missing. I've given up using them.
@Elnnina I wonder if a water erasable marker would help you keep your hem stitches where you want them, regardless of how distorted they are by stretching. The blue ones that you can get usually wash away completely, so long as you don't iron them.
Not having a coverstitch I would use a triple zigzag stitch for long sleeved vests. Possibly something to consider then try an easier stretch fabric for getting the hang of your coverstitch. Cotton jersey and interlock tend to be much better behaved.