You would click on the prune button at the top once you have searched for those members. I would advise ensuring you backup before a mass action like this.
You should not actually change the core files. If you wish to use your own error file, you can do this by adding a constants.php file, and pointing to a new error file. So you would add something like
<?php
define('ERROR_PAGE','newerrorpage.php');
I can see that happening but cant see why. I have created a ticket on this for you, and escalated this to our development team for further investigation
As mentioned in the message there, its likely these have expired, and the message there would indeed be valid if this is the case. Was this sent out before or after you switched to ourselves?
I can see this happening on your site and will need to get this escalated. I have created a ticket for you for this reason, and will get that sent to our developers to review
There isnt a way in which to do it within a forum. If you have it showing as a preview it will indeed show a preview of the first post. As mentioned above, you would be best to use a stream for what you are looking to do there. You can see a guide on how to uses these here
I would need some examples of this happening on your site in order to investigate. I have created a ticket for you so that we can go through in more detail to see what is happening. Please respond to the email notification of this with some members and example posts where they are not yet validated. This isnt something that should be happening, and I have not seen happening elsewhere