Members being on the end is easy and if you don't mind spending a couple of minutes getting your hands dirty the entire order you want can be done.. Members appears after all tabs inserted into the middle position.
All plugins insert tabs like this:
Code:
$extraTabs['exampletab'] = array(
'title' => new XenForo_Phrase(''),
'href' => '',
'position' => 'middle',
);
Editing the position in the listener will determine which of the primary positions it will appear.
The xml file that installs the plugin has an execution order for this very call.
Code:
<listener event_id="navigation_tabs" execute_order="10" callback_class="My_Example_Listener" callback_method="navigationTabs" active="1" hint="" description=""/>
The execution order determines when this is called to insert the tab and there for which tab it appears in front or behind of its particular section (home middle end).
Most of your plugins probably use end, if you change their listener to middle they will appear before members. If the plugin author gives you the option between the three positions you would also be able to roughly move it to a different position.
It is possible for any plugin to dynamically change their position to home middle or end. They just can't sort themselves in the scope of those 3. Thats the execution order which is edited in debug mode.
Moving Forums is a different story its more of a template edit. For your scenario move Members to the end section, Forum to the position members was in. Events and Chat's listeners need to be set to end and have Gallery set to Middle which will appear in front of forums if moved to the current position of members.
Xenforo uses an if then to populate the html for output based on what was already inserted into variables via code. Xenforo doesn't use an if then to determine order. The order was already determined by the time it comes to template statements. You can physically move forums in the template and it should appear where you put it. Which I recommend as the current position of members that way all middle tabs will appear ahead of it.