As far as a disarmed mode that is in effect just not having a Active mode in Arlo Assistant. Part of the setup for Arlo Assistant is to set your Smartthings mode to be active, but to also set the mode to take no action or notification when motion/audio is detected on the cameras. This in effect is a disarmed state. If you wanted to represent this in Arlo Assistant you could have a mode that is triggered by whatever in Smartthings you want to represent Disarmed, but it isn’t needed. SHM integration only works with the Classic app. You can use virtual devices in the new app though with STHM, and then use that virtual device with Arlo Assistant to integrate with the new app.
It is just a smartapp that uses the official integration. So if the official integration is having a problem then Arlo Assistant will be impacted as well.