No, the router is not compatible with AiMesh and the antennas cannot be unscrewed. It does not support external antenna connectors and has internal antennas, while ZenWiFi XT8 is AiMesh compatible, but there is no direct compatibility with this model. The available data does not fully cover this, check our Community for more details.
Yes, it is possible to switch off the 5 GHz WiFi in the software. You can do this via the WebGUI or the ASUS Router App. In the WebGUI, go to "Wireless" > "Professional" and set "Enable Radio" to "No" for the 5 GHz band. Alternatively, you can make the appropriate settings in the app under "Settings" > "WiFi" > "Wireless Settings" > "Advanced Settings".
Yes, you can switch the Wi-Fi on and off via the software. ASUS routers usually offer a user interface that allows you to control the Wi-Fi functions. You can do this via the web GUI or the ASUS Router app. In the app, you can change the settings for the Wi-Fi, including switching the connection on and off.