The immediate feedback to your idea is to ensure that the platform itself is not already considering an app development, as content might disappear for your app if they promote theirs. Equally, later on, assuming you’d demonstrate success with this app, how do you protect access to the content it builds on? You need to construct some barriers of entry short and long term for your idea to keep it viable.
Regarding your specific questions:
1. Confidentiality or Non Disclosure agreements are usually in place before work starts with contractors, such as app developers. Here’s a template for reference: https://simmondsstewart.com/templates/mutual-confidentiality-agreement/
2. Yes. A lot of founding teams start like that (sweat equity rewarded via shares in the newly formed Co), but you want to ensure that the working relationship is tested and remains strong because that’s where all could come undone if it doesn’t pass the test of time or hardship.
3. Bank loans make sense when you are much further ahead (i.e. when you have strong and reoccurring revenues) and after you have looked into a number of other capital sources. Bootstrapping or developing a prototype in your spare time, with co-founders who are usually putting in their expertise for sweat equity is how most startups begin.