On the handsign tip:
I think it's possible that some NPC members don't realize they're "imitating" NPHCs. I'm just being blatantly honest here, because some NPC members know little to nothing about NPHCs. We had no NPHC orgs on my campus until I was a senior. Basically the only reason from my relatively competent knowledge of NPHC is my involvement in Greekchat. I am almost positive that that's often the case for other NPC members, even on campuses with active NPHC groups. Unless, as DSTCHAOS mentioned, there is a good Greek Life office that encourages interaction between various councils and provides PR information on the activities of one to the members of the other.
I've often seen images of NPC groups making signs (ADPi making a diamond, KD using two women to make a KD, Tri-Delt making a Delta or triangle). It's not entirely clear to me that these women thought "Hey, NPHCs have really cool signs, we should do it too" which certainly would preclude them thinking "Hey, I should consider how this hand sign will look to NPHC, which treasures hand signs as part of their institutional identity." They might not even realize that NPHC does signs. If they do, they might not realize the important role NPHC handsigns play and just assume that it would be viewed as a tribute or "that's cool" type of thing by NPHC. I mean I could especially see this in the case of Tri-Delt making a Delta, for obvious reasons. I just don't think the majority of NPC members would make the connection... which is a sad testament to NPC members' lack of knowledge about NPHC... but in my experience, that's the way it works.
