The ordering of the columns isn't important.
As for updating the other features, the latest version for both Joomla 1.5 and 2.5 have the same column structure. Upgrading your J1.5 site first and then going to J2.5 should work. The J2.5 installer will also easily update your J1.5 installation as well.
So, either way, when you install the v4.3 release (on J1.5 or J2.5), your database should be automatically upgraded without issue.
Let me know if you run into any issues though and we'll gladly help,
Alex