First: it will be this draw a decision under the same user? It is not entirely clear how you want to use(in the interface of Bitrix, either as a separate app in Bitrix)
In General, the bitrix24 app is just an iframe opening the link. Ie you on your hosting place the script/page with the app and Beatrix on the link it pulls up, as if you are this page in the browser opened. Examples of applications are in the documentation, take, try, understand)
1. It is added through the CRM - settings - properties of the goods. Or through the api https://dev.1c-bitrix.ru/rest_help/crm/cdeals/crm_...
2. Work with jquery as usual, nothing special. But to get the dom elements via jquery, Bitrix will not work if there is no box.
3. Not entirely clear what you want