I just went out to the shop and solved the problem, which was that those optoisolators needed 24v rather than 5v power...I just checked the voltage across the estop input and got 26v, then checked the resistors in place on the BOB, and sure enough they were 2.2k which is what was called out on the BOB schematic for 24v inputs. Now all is well and homing is working perfectly. I should have looked at the resistors first but what really threw me was the fact that the BOB input LED lit up when operating the switches; now I know that is merely an indication of a signal, not necessarily an ADEQUATE signal! I'm lucky I already had no hair to tear out..
thanks for all the help!
Tom