At the present time, no custom code of any kind can run locally. Smartthings was originally designed as a cloud-based architecture, and it still mostly is. The problem that they have is that anything that is going to run locally has to be downloaded to every customer as it becomes part of the hub’s firmware. They just don’t have a way to let an individual customer run their own code locally right now. So the only things that run locally are the device type handlers that the company has selected to give to every single customer.
As far as what to do about the Osram bulbs when the Internet is down, as long as this is something that doesn’t happen very often, you just need any switch that can control the power to the bulbs. Then in an outage situation, use that switch for on/off. But you aren’t going to get all the features when the Internet is down.
If you take SmartThings out of the picture, The Leviton can operate locally and the Osram Lightify bulbs can operate locally, but there’s no way to use the Leviton to control the Osram bulbs.
If you want local operation with SmartThings, you have to stick with devices that use the official device type handlers that have been confirmed to run locally.