I do not think this is possible. Offline, with sqlite, yes. But online, when there are constantly new values coming in, the new values would lead to problems again. So, unfortunately, you have to calculate the sum as well (using negative values is totally fine, see somewhere here and also in the readme of the integration. In the backlog there is a feature request to automatically calculate the sum based on the last existing value, this I plan to do sometime.
This would be possible for sure, I only do not know how. My integration just offers a HA service, and for file upload I probably would need a web server. If somebody has a better/simple idea, please let me know. But uploading the files can be done e.g. with Samba or via ssh/scp, so, as you said, thats a minor thing.
Sorry,
Klaus