This may be a two-fold question, but I think I got an answer on the first part, which then leads to the second part:

In order to keep the strain of running an "detect updates" function, as low as possible, I would like to make it as streamlined as possible. But there doesn't seem to be an equivalent to ex. "bpy.data.movieclips.is_updated" (for the motion tracker clip) for the sequencer? 

So I guess the only way is to compare the currently active strip's data with the data of a previous active strip, but how do I do that?  

    import bpy 
    from bpy.app.handlers import persistent
    
    global oldStrip
    oldStrip = bpy.context.scene.sequence_editor.active_strip
     
    @persistent
    def sequencer_update(context):
        #if bpy.data.movieclips.is_updated: # Updates on tracker file updates. No sequencer .is_updated function? 
        activeStrip = bpy.context.scene.sequence_editor.active_strip
        print("Active strip: "+str(activeStrip.name))
        """ # if oldStrip != activeStrip: # 
        if oldStrip.name = activeStrip.name:
            if oldStrip.frame_final_start == activeStrip.frame_final_start and oldStrip.frame_final_end == activeStrip.frame_final_end: print("Strip not moved, maybe a rename or other property changes?")
            if oldStrip.frame_final_start != activeStrip.frame_final_start and oldStrip.frame_final_end != activeStrip.frame_final_end: print("Entire strip moved")
            if oldStrip.frame_final_start != activeStrip.frame_final_start and oldStrip.frame_final_end == activeStrip.frame_final_end: print("Start handle moved")
            if oldStrip.frame_final_start == activeStrip.frame_final_start and oldStrip.frame_final_end != activeStrip.frame_final_end: print("End handle moved")    
            oldStrip=activeStrip                  
        else:
            print("New strip selected") """    
            oldStrip=activeStrip
        
    #Register 
    def register():
        
        handlers = bpy.app.handlers.scene_update_pre
        for handler in handlers:
            if ("sequencer_update" in str(handler)):
                handlers.remove(handler)
        handlers.append(sequencer_update)
    
    def unregister():
        handlers = bpy.app.handlers.scene_update_pre
        for handler in handlers:
            if ("sequencer_update" in str(handler)):
                handlers.remove(handler)
    
        #Unregister operators
        bpy.utils.unregister_module(__name__)
    
    if __name__ == "__main__":
        register()
    
    #unregister()