In Safari, how can I allow a site to locate me, after I have refused?
I will try to answer in the following order

3 - Settings are remembered for the whole site. Not for the page. 

2 - There is a settings under Preferences -> Privacy, for this. You can select either to be prompted once a day, only once or deny all the time. 

1 - I don't think there is no way to do this directly. But I think you can work around it by changing the settings in answer 2 to prompt daily, visit your website, allow locations when prompted and disable the settings back to prompt once if you want to

More on location services https://support.apple.com/en-us/HT204690