On Apple devices that wouldn’t be an issue because you you can load the Twitter Web interface into a real Safari and get the markup from there for processing. Twitter can try things like CAPTCHA but this will degrade the experience for all Twitter users and you can simply delegate this CAPTCHA to your user to solve anyway.

While for reading that works, there is an easy why via posting to spot people use that app. They can change at any moment the input names so that when posting a tweet you are exposing that you are using the scrapping client.

Remember that you are loading full Twitter website in the background. You will use their original interface to post Tweets.

Yes, but the app is not a human and will likely interact with the website in a more predictable (and thus, detectable) way than the human itself would.

But you also have a human using the app, you can transfer the interactions with your app to the Twitter's Web UI.

Apple wouldn't permit such an app in the store so it's a moot point.

Why do you think that Apple wouldn't permit it on AppStore?

There are already apps like this on the app store

If something is built as a browser extension and the requests it sends look like the browser would normally then it must be hard to detect.

The requirement for the extension/plug-in has to be that: it looks like someone uses Twitter normally.