That does sound about right, and should work in principle.
With approach 2 only though, you are consigning cushions to 'subordinate/child items', also I don't know how it will work with your existing cart software.
also won't it prevent customers from buying cushions only ?
The reason I mentioned part 1 is that it should, in fact, be easier to implement, and with approach 2 as well, you get the best of both worlds, cushions (or whatever accessories you happen to want), available in their own right, and as subordinate items.