2) you can simply add another dummy model to the new page of the same App. You have to create new model and a page on the existing App.
On this page place the required maintenance info.
Then whenever you have to do maintenance you can put your main model to offline mode (then only you and everyone who has Admin will be able to work in the model) and open the maintenance module for users so they could see the page of maintenance.
Re: How can i add maintenance message page when my model under maintenance
If you have only classic UX then you can create a role called Maintenance mode and a DB with the info. And provide access to that DB only. Then whenever you have to you can change users' access to Maintenance mode and users will see only this DB.