ad 1) Once you have graduated and you are not employed in the lab anymore nobody can force you to do work.
That being said there are a few options for you that might be interesting (but it depends what you actually want):
If you are interested in a publication record then agree with your
former PhD supervisor that you co-author all papers you gave support
The amount of remote help you supplied sounds rather excessive from your description. It's not unusual to get the occasional email about specific aspects of your previous work from PhD students that continue with related work, but your description goes very far beyond that.
In general, you're not obligated to work for free. If it is of interest for you, ...