The strange thing here (speaking as a VirtualBox developer) is that when Additions are installed VirtualBox doesn't actually grab the mouse so there is nothing to release. I assume that this is always with Synergy running on the host system? Have you tried talking to the Synergy people to get their point of view on the matter? Knowing what Synergy thinks is happening might be helpful in solving this, and they are best placed to help you find this out. (And sorry I haven't had time to follow this up, but I'm afraid that I have an awful lot of things to do with a higher priority, so that I will only be able to do something here - if at all - if other people are able to find out precise issue rather than just the symptoms. I can certainly give a few hints to help you along the way if you are willing to put an effort in.)
