Yes, yes, and yes.
Your alert action is likely already configured if it works with BI motion as is. You do need to check ONVIF and Extern for IVS events in both trigger and alerts. No changes should be needed to AI (it doesn't care where the trigger came from).
"required objects": we'll assume you mean the setting found in the alert tab, "on alert" action settings (for example, "required objects" setting for a text message action). If you leave this blank, all confirmed triggers (based on "to confirm") will result in an action. If you put something in required objects, you only get the action if AI confirmed those objects. Think of required objects as a secondary filter for more granular control of push/text/etc.
"Check the trigger": we'll assume you mean the trigger tab/AI setting "To confirm". Consider this as your primary alert filter (gets past here first before required objects come in to play). If you leave it blank, all triggers will result in an alert (sent to the secondary filter anyways). If you put something in "to confirm", it won't alert unless it sees that object.
Yes, to use ONVIF events, in the camera trigger tab you should uncheck "Motion Sensor" (which is BI's internal stuff), and check "Camera's digital input or motion alarm" and "trigger until reset". That basically makes your cam IVS do the job of BI motion. No changes to AI are needed here. However, you may also need to check the "ONVIF" and "External" boxes in the Alerts tab.
Lastly, Dahua cameras can have a few gotcha's when it comes to using IVS events. First one is some versions require configuring a second "onvif" user with admin access; it simply won't work with the default admin account. Then often you have to change your camera video settings in BI, to use "Dahua" type with port 8999 for onvif, instead of Generic ONVIF/RTSP with port 80. Lastly, sometimes you do all this and it doesn't work due to improper sequence of changes (camera then locks the new user account). If this is the case, you have to first delete the cam in BI, delete the onvif user in the camera, recreate the user in the camera (save it), then readd the camera in BI. Lastly, when you change video settings, don't forget to redo your PTZ config if you use it for API control (BI annoyingly resets this whenever you change video settings).
Good luck!