You should be able to add/delete locks codes using the Smartthings Lock Guest Access smartapp.
As far as lock codes being deleted and continuously updated, that is a bug in the stock Edge driver that has not been addressed by ST as of yet.
Your lock is supported by [ST EDGE] Z-Wave Lock PH - Devices & Integrations / Community Created Device Types - SmartThings Community from @philh30. I would suggest trying his driver and see if the deleting/updating problem goes away. I know he specifically addressed it for Schlage locks. If that still doesn’t fix it, he may be able to add whatever logic fixed it for Schlage locks to fix it for the Kwikset locks.
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