Hello,
1) It’s our issue. We’ll fix it in next theme update. If you provide me admin panel and FTP access to your site we’ll fix it on your site.
2) This is default WP function. Looks like something is wrong with your WP installation if you get such notice
3) There is no such hook in our theme. Very likely it caused by some plugin. Make sure that all the plugins are up to date. Disable them one by one to find which one causes the issue.
Regards