Yes. To re-activate your account, you need to follow the instructions in the email you received. There are at least 2 ways to do that.
30 days. As it says in the email.
The v3 scheduler has been included in Anki builds since 2021.
The AnkiDroid version that you’re using came out in 2023 – and in fact was the last version before AnkiDroid switched to v3 enabled by default. You’re smart to be cautious of those warnings, but I can vouch that I personally used that version for many months with the new backend and v3 enabled, and had no problems.
Your desktop Anki version is a bit older than that – but it should be fine to enable v3 there as well (in Preferences).