For web access, stick it behind a reverse proxy and use something like Authentik/Authelia/SSO provider of your choice to secure it.
For full access including native clients, set up a VPN.
Not at all.
Lemmy is overwhelmingly militantly anti-Tesla, which is understandable considering who owns it, but it does mean that users tend to interpret any neutral or factual statements (basically anything that is not outright criticism) as having a pro-Tesla bias.
In this case, all I am stating is the fact that this specific change currently only affects corporate users. That could of course change in the future.
There is a rich history of cloud based data providers pulling the rug from under users with no warning. Look at what happened to Nest users when Google took over.
Something to note: Tesla has two vehicle APIs, the Fleet API for commercial accounts and the Owner API for individuals. This change currently only impacts the Fleet API.
If you are an individual owner who accesses your vehicle data from the Owner API (usually via a self hosted tool like TeslaMate), this does not affect you. Yet.
Right now none of the native clients support SSO. It is a frequently requested feature but, unfortunately, it doesn't look like it will be implemented any time soon. As with many OSS projects it is probably a case of "you want it, you build it" - but nobody has actually stepped up.