Hi,
Things don't start "not working" all of a sudden for no reason. Someone must have changed stuff on your website / web server.
1. Go in the menu System>Emails and make sure that the "order creation notification" and "order status notification" emails are still enabled. Otherwise, that would explain the issue. Also, make sure that the orders are confirmed in your backend. If that's the case, then HikaShop is sending the emails to your email server through the Joomla mail library. So the only thing left is that your email server is not sending the emails for some reason and looking at the log of the email server would help understand why the emails are not sent by the email server.
2. So where are they redirected to ? What is the URL ? Are they redirected at all ? If not, that would mean that the auto redirect setting of your PayPal merchant account has been turned off.
3. This error indicates that the HikaShop google analytics plugin has been activated and configured with an invalid google account id. You can check the settings of that plugin via the Joomla plugins manager.