nest the picture that you want to move around.
that way you are fooling the track matte key into thinking that the nest is the picture.
you can then move the picture around without moving the matte.
the best way to do this is: make your nest 2 layers. on the bottom layer put your pic. on the top later put your matte.
apply a 50% opacity to the matte. move your pic to your liking. when done make the matte 0% opacity.
won’t you have to remove the transition after you have made the trim?
i usually place the clip on the right on an upper layer and do the trim there.
sometimes you just have to embrace new editing software and move on.
i had to do that when moving from FCP7 to Premiere concerning track matte keying.
FCP7 was a snap. Premiere not so.
you could send a “feature request” to Adobe.
i think that if you click the little icon in the timeline window that is shaped like a stacked layer, when you drag your sub sequence to the time line it will not be nested…but the layers might not be stacked the same way.