Hello,
Point #2 is implemented. See the documentation:
https://en.numista.com/api/doc/index.php
This works only to authenticate yourself, as I would like to avoid third-party application to store the Numista credentials of other Numista users. Desktop applications like OpenNumismat are able to use the current "authorization code" authentication.
Feel free to share feedback.
Point #1 is definitely interesting to implement in the future.
Point #3 would be nice to have, dates could be added both for the currency and for the ruling authorities.
I don't intend to implement points #4 and #5.