A well thought out plan. Not so sure about the subdomains thing etc. But then again I don't use anyway. I usually stick to making sure I have a good tar backup of all files in the DOC_ROOT directory and a SQL database backup. This should be enough to restore your site in minutes.
Regarding the upgrade: the most difficult bit is always upgrading your database schema. This you must do by executing the different upgrade SQL scripts in consecutive and correct order. The rest is just copying the 1.9 Tiki files in place and possibly empty your smart cache.