Hello

Second problem I see.

Posts that have been read as a guest are not acting the same as a user. When I read a thread as a guest then click 'back to forum index' the title of the thread and replies is still bright blue

Are you reading the forum in the nested view or every entry on the single-posting-page?

Using threaded view, opened the complete thread. In 2.4.2 If I do the same as a user then click 'back to forum index' the thread title text and reply text is darker blue and the red icons no longer show which shows that you've read the posts. I thought this should be the same for a guest user too?

As a guest opening a complete thread then clicking 'back to forum index' the thread title text is still bright blue and the icon and reply icons are red still.