Hello there, I've been following this thread closely because I have a Unchained - Protean Eidolon (Snake Constrictor base).
I didn't realize every time I made a Maintain Grapple I would do the Constrict damage again.
I just wanted to point out to rabbedrabit to keep an eye out on the pre-requisites. You said you were level 3? There are a lot of requisite feats for Greater Grapple including 2 feats and BAB +6. And Swallow Whole isn't available until Level 9.
Of course it is possible that you are retroactively seeking info on an ability you've had for a while and are trying to figure out how to use.