I’m pretty sure that neither 3 nor 4 can be done via the API.
I’ve requested before the ability to interact with tags via the API. But as things stand, it is not possible to query tags or set tags via the API, at all.
I’m fairly confident that the maximum number of safe days is also not accessible via the API. Not only do the API docs not mention the existence of a field containing the maximum number of safe days, a look through the JSON returned by the API for one of my goals does not show any field with a value that’s the same as the number of safe days on that goal, confirming that it isn’t just contained in an undocumented field.
Not receiving this value when querying the API strongly implies that it can’t be set through the API either, though I guess you never know. But as far as I can tell, this is indeed lacking.