#1 - you are right, but it's hard to get to the end user... I'm not sure if it would be easy to anyone to get into an advanced logging option. Seeing straight that you are on https in whatever area of the website feels warmer. Also, I've heard that Google likes https-based websites more than regular http ;)
#2 - I suppose, SSL also protects from phishing the browsing history, so it's not just about logins; however your suggestions sound reasonable and worth of consideration...
#3 - yes, ssl would be incomplete if some file are loaded from non-secured domains, and forum owners should always take it into consideration. As you wrote, ideally https forum owners should allow to include images only from https websites, or allow local file uploads. Not sure how we improve this here on miniBB forums, probably I will come up with some simple solutions :)
#4 - yes, it takes time, but nowadays not so long... so this is not quite important.