Stuck and stiff sora front derailleur I have a used 2011 trek 1.2 compact road bike with a stiff Sora FD-3400 double front derailleur.

Shifter seems very stiff, and when I shift above the second index (trim or third) by accident it gets stuck on the big chainset ring and then it takes a huge amount of force to release it or even impossible and I have to release the shifter cable from the derailleur. As I understand I should set the top limit so it won't go over the second index right? But even if I shift to the second index and try to lower to the first it takes some heavy force 
and there is a loud clank when it does get released. 

I think its the derailleur because when I detached the derailleur, pulling the cable in the housing seems smooth, and the shifter has good and smooth pull and release for all its indexes and trims. 

Here's a video of me pulling hard on the cable 

                
            

Here's my video of how hard it is to move: 

                
            

Compared to what I see here at 1:34 mark:

                
            

when it does shift this is the sound it does at the end of this video (not my video) 
                
            

I also suspect the previous owner had something to do with it, since there is a little dent under where the derailleur clips shown here:


Am I right thinking its the derailleur and should replace it? What else can it be that  I should check?
My SORA front derailleur cable broke (after gear change difficulties for several tens of kilometres, and ineffective cable length adjustments). I replaced the cable, but I still had to push the gear change lever very hard to change chain wheels (which is when I found this web page). The culprit was the plastic cable guide screwed onto the frame under the crank (GIANT Cross City 3). This guide had been cut through by the front derailleur cable and the cable had started cutting into the frame. After replacing this plastic guide ($3.00), as well as the cable, changing the chain wheel is almost as easy as when brand new.