I agree, something so basic shouldn’t require a work around. I wish Avid would rewrite their effects interface. I feel they’re stuck with the way they decided to do it 20 years ago.
But another workaround is to use the paint effect and a matte key:
1: Layer your background on v1 and your intended foreground on v2
2: drop a paint effect on v3
3: In paint, draw your shape in black around the part of the image you want to key over the background.
4. Still in the same paint effect, draw a white rectangle over the whole image then send it backwards, behind the white shape. You’ll then have an image you can use as a matte key.
5. Alt drag a matte key effect onto the paint effect on v3 to nest it.
6. Alter the position properties of the matte key. You will now see the foreground layer moving without affecting the background.