Limitations to MIT app inventor - before starting



I would like to make an app for the event I am going to organize for my non profit organization. (The event is to raize funds)
The idea is:
Main function: 1 to 1 chat app, where all the users (around 1500) can chat with each other. It is a party, every entrant gets a sticker with a number, so the idea is you get to know people by sending a message to the number. I wanted to do this by creating 1500 users created beforehand, including a password that they get with the number (so people can not act like they are an other number).
I would also like to make a few other pages, but they require little coding.

The reason I post this before starting is not because I need a guide on how to do it (I enjoy making the app) but because I was thinking there might be limitations to MIT app inventor.

My concerns are:

    1. Is it possible to make the users and their passwords beforehand
    1. Is it possible to build the app so it can be downloaded both in Playstore and Appstore?
      I found this: [Building Apps for iOS with MIT App Inventor](https://ios buildservertest) but when I checked all steps, I already noticed this is missing with me:
      my screen: image
      tutorial screen: image
    1. Is it possible to have 1500 simultanious users on an MIT app? Do I have to set up my own server with my laptop? I did some research and found something about redis servers. Would it be a solution for me to set up a redis server? I don't really understand how apps work concerning servers, and databases and what the impact is of many simultanious users but I thought maybe I could have a DOS error.
    1. Do I have to mind about safety of the chats? I wouldn't want other people to be able to access all private chats.
    1. Do I have to take certain steps in advance, if I would want to have updates on the app, without all users having to download it again? (The event comes back every year.)
    1. Any recommendations, problems I didn't think of, suggestions are welcome!

Thanks a lot in advance!

thank you for your quick response.

Unfortunate for me. I ll try to build my app with flutter.