1. Yes. Especially for free tier AWS server, because it is basically server that you can install anything on them including relay software. You can set it to only accept your nostr pubkey event when write https://github.com/Cameri/nostream/blob/main/CONFIGURATION.md
2. Yes. You can use https://github.com/MutinyWallet/blastr as broadcast only relay.
3. Yes. Anything that can help directly or indirectly nostr protocol. Relay is the basic component.