I think your lack of atomicity is going to get you into trouble

Or I would if MySQL didn't do the right thing concerning LAST_INSERT_ID - it is maintained on a per connection basis, so unless I change connections or do some equally silly I should always get back the right ID. See. 8.1.12.3 How Can I Get the Unique ID for the Last Inserted Row? for more info.
HTH

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broquaint