1. are there any structural differences for the mysql tables for 1.8.1 between a Joomla 1.5 and joomla 2.5 installation?
No.
2. Can I manually copy all the CB tables from the 1.5 site to the 2.5 site for the custom fields and user lists I have created?
If user ids are unchanged, yes. You will need to go around your install after doing this and fix any ACL issues as usergroups and view access levels have changed in J2.5. This means checking tab access, plugin access, moderator group in configuration, etc..