Okay, so multiple things here.
You have to call “dial” on the created “;1” channel for the “;2” to actually go anywhere. This is the way things work because it’s a two stage operation. Create lets you set things up and add it to the bridge. You’re not actually going anywhere or dialing until you dial. Did you do that?
Secondly on originate the “;1” won’t go anywhere until the the “;2” leg is answered. Did you do that?
Oh, thirdly, the “;2” will always go to dialplan because that is its purpose. It dials an extension and context in the dialplan. The “app” specified to originate is for the “;1” leg only.
As for subscribing originator, I don’t know with any certainty but probably to get events as they are related. Even if a channel isn’t in ARI you can still subscribe to receive events about it to know what is going on.