1) This can be changed easily in the Form editor. I will but It on my todo list.
2) This is not so easy to implement as a form does not know what the previous form was and you can not jump back to every form. Maybe we can add a second save button that did save the result without closing the form in a first step.
3) This should not be a problem.
5) On this idea I'am workin already in ISPConfig 3. The server part of ISPConfig 3 is completely based on plugins, so that the core does not contain any functions that modify config files etc. This makes it easy to support a high number of different MTA's for example by simply symlinking the needed modules in the style of the mods-available + mods-enabled directories that most apache2 confihurations use.
But Backporting a part of this to ISPConfig 2.3 might be possible, at least to provide event hooks for some events like user_insert, user_update etc.