1) the law, at least in the US. Laws such as copyright and maybe even the CFAA have been abused to outlaw adversarial interoperability. You'd need to either practice good opsec and operate anonymously or be based out of reach of US-allied law enforcement.
2) Apple (and maybe Google too?) - they will remove any client for a third-party service unless said client has explicit approval from the third-party, all the way up to ridiculous levels such as rejecting an app using the LAN API of smart home devices explicitly designed for this purpose: https://community.lifx.com/t/app-store-rejection-permission-...
On two, it seems like they are concerned with trademark to me. A workaround could be making a website that parses and displays Twitter content and call it something else. Then, in an app, make no mention of Twitter and only the website.
this is good, but it would also be helpful if you supported the anti DRM movement. Some people have developed ways to get around certain DRM such was Widevine, from dumping your own CDM to Widevine proxy. Just ignoring the problem is not going to make it go away. Over the last two years DRM use for streaming content has increased significantly. If you want to really help, I would look into contributing code to these projects, or donations.
It does nothing to dissuade content gatekeepers from employing restrictive DRM on their sites.
Anti-DRM would be avoiding anything that gives money to those that employ DRM to incentivize the removal of the DRM. Frankly, flat out piracy (streaming ripped content) is more likely to result in the removal of DRM than making it appear that the DRM is working well for the provider.