Those covers do look nice! Did they put the clip on your armrest correctly, i.e. matching the original?
I know what you mean about the tricky plastic clip. I had my assistant mechanic (aka wife) help compress the foam in the armrest to give me a little more slack while I worked on the clip but it was still a major pain. And mine kept popping off afterward until GAHH sewed it on.
If your armrest removes easily like mine, I would take it off for installation so you're not performing back seat calisthenics (not there's anything wrong with that in other contexts). If all else fails, try profuse cursing.
You might also give GAHH a call and see if they have any suggestions, maybe there's a trick to it.
Good luck, keep us posted!