Based on how you worded the questions, yes; however, not allow of the scenarios you can do between current shipping IP phones are equally possible from Jabber. For example, you cannot currently move a call from an IP phone to the Jabber VoIP client as it can't resume a held call from a shared line appearence (this is the inverse of your second question). CUCM supports it, the client just doesn't know how to do it yet. You would have to use Mobile Connect to move it to the mobile identity/cellular number.
Jabber for iPhone - call handoff - use softket to transfer call from client to mobile number (Jabber for iPhone 9.5)
Jabber for iPad (jabber video) - call handoff - use softket to transfer call from client to desk phone
This depends on the desk phone's shared line support, not Jabber Video for iPad. In this scenario you put the call on hold from the iPad and the desk phone will see it as a held call that you can Resume.