1) Yes it is possible -
2) Yes it is possible
3) No you can stand up a new vCenter 5.5 and copy the database over - you might want to take a look at the upgrade guide as it covers all these possibilities - http://pubs.vmware.com/vsphere-55/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-55-upgrade-guide.pdf