AFAIK Musk doesn't (currently) own anything that could counter AWS stopping hosting Twitter. Starlink comes closest, and it's really a different thing. Regardless of the trend, I'm not sure it's easy to counter stopping hosting Twitter just yet.
No, but Google is tied up with Starlink in offering ground infrastructure, so that makes it unlikely that Google will go against Twitter. MS also has been promoting Azure Space which is supported by Starlink. Musk has enough connections to big tech, so unless they change their minds, or AWS and Bezos goes against Elon solo, odds are he doesn't have to worry too much.
Companies have been onto this for some time, especially social media, and have been building their own completely separate ecosystems from big tech. Gab has it's own servers for example, and Rumble right now does more business with it's servers than it's video platform, just because it's separate from big tech and therefore offers very little risk. Rumble has been acquiring and is growing quickly and recently got some huge investments into the company. The management at Rumble are also big fans of Musk.
If AWS wants to cause considerable headaches for Twitter asap, they can, but in the long run, Twitter can remain, and directly attacking Elon like this is asking for trouble. It's not just Elon who will feel attacked. The internet, is supposed to be about openness and working together, not closed doors and monopolies.