Thank you for your post. I see this guestbook plugin is being used more for all kinds of usecases, so it is good to make it more customizable.
About the “write a new entry” you can change that already in the settings of the main guestbook plugin.
About the “signed on” text, it should be possible with 4.1.2 which was just released. Try this CSS:
content: 'signed on ';
The colons you mention have been removed.
Does this help you?