yeah... I was going to post and ask if Zangetsu always only had 1 arm.
I can only assume Zangetsu left a piece of himself(the hollow side) inside Ichigo.
as for zangetsu holding back the hollow, when Ichigo has dominion over the hollow, Ichigo is in control. when the Hollow is dominant, Ichigo has to try and not get overtaken by the hollow and transorm like he has the few times he has. So, I assume with no zangetsu in Ichigo, that automatically puts Hollow Ichigo in the reigns of his spirit power.