Never worry about adding appropriate bonus feats to a mindless creature if it still could have qualified for and taken all of them if it had an Int score. You're just giving back what was taken away for (in this case) negligible benefits (immunity to mind-affecting comes from the type anyways, and Int -- provides little else).

The Marilith ability to use everything as a primary hand attack is a great idea. I didn't see the discussion of it when I posted before, but I heartily agree with it.