1. The Forum Module used the installed Editor, where content images are handled.  So this must handled over the Provider.  If you are using the Standard FCK Editor
which is delivered with DNN it is a FCK-Editor Provider issue.

2. If I understand right you meant teh content too, so this is caused on the FCK editor too.

3. The FCK editor didn't have a standard settings for the images in the content.  But you can write a css style for it and add this style to the editor, the user then have to set this style for images.

The only Forum image handling are the avatar uploads so all of your descript issues are on the FCK- editor.  I am not sure at time if we can override the handling of the FCK editor.  A other problem will be if we found a way the Forum Module will not running if you are using a other editor. 

But I think you can report your issues to the FCK Editor Provider Forum, a better handling on existing file names will be great on uploading files via FCK Editor.

The problem is that the FCK-Editor is a 3rd parity module (open source), and can be changed to some others.  DNN only provides a Provider to use the FCK Editor. I am not clearly with that code, and if it be posible to hack this.  Sorry.  But a hack from DNN is not a valid solution, about the FCK-Editor will brocken then if the Forum Module is not added to the Portal about missing control.  so it only can be a private hack for you only.