I know you and I had some discussions about scripting before in the who submitted thread. You weren't underselling how densely packed you write your scripts, that's for sure!
There's a lot here, and really the story itself is pretty much fixed, since it is a different POV of a scene from the book already. After panel one the reader knows the beginning and the end. So in this case it's all about how you get from point a to point b.
Now onto a whole bunch of rambling that is pretty much completely subjective, and may or may not be useful for you at all. (Opinions being what they are after all)
My first reaction, was there's an awful lot of tell, and not near as much show in your script. Let the visuals tell a portion of the story for you, it's one of the things comics really excel at.
On a pure structural note you have 2 pages of opening, a single page of turn, then a single page resolution. I think to really make this scene pop, the turn sequence needs to be the bulk of the story. I think it would be relatively simple to expand the Vietnam flashback sequence. It would involve some editing and serious streamlining of dialogue and captions especially in your opening. But I'm certain you can get there and still be telling the same story and maybe even amplify the drama a fair bit. You can maintain the inner dialogue captions talking about his current situation as you show the president being active in the flashback panel calls. You could likelydo a lot playing off the juxtaposition of the President fretting over his helpless situation. He's facing his own senseless execution just like the family in his younger days.
Probably the easiest means to do so would be to cut some of the panel calls that are "Same as previous panel, but some small addition". (One example was a wide shot of all four characters. Then the same wide 4 shot, but now Nemesis is holding his pistol up) You can very easily boil this down. Take advantage of the gutters on the page and let the reader fill a few of those gaps in in their head as they're reading.
I used the example before in a different crit. But when I first started writing scripts I had a habit of writing, Panel 1 character stands up. Panel 2 Character walks across the room. Panel 3 Character puts on his coat. Panel 4 character opens his door. etc. etc. The same story can be told by saying panel 1 Character standing up. CHARACTER: TIME FOR A WALK. Panel 2 Character in a coat walking in the park. You get to your destination as a writer and you save a lot of real estate on the page for the sequences that really matter. There's also the small added bonus of your future art partners not having to draw the less interesting in between bits. Unless they are 100% vital to the story. Especially with only 4 pages, you've really got to kill your darlings, and really cut and refine the script until it is razor sharp.
Hopefully I've said something in this ramble that resonates for you. Good luck to you moving forward and keep working hard.