So, this being a duel - what do you suggest the LA should do instead?
If you watch the film, the La-7 reverses immediately after the merge ... because he probably considered that when 'dueling', his opponent would also reverse to re-engage for another merge.
If I were the La-7, in the MA, I would have looked out the rear after the merge and when I saw him extend and climb like he did ... I would have continued a straight flight line and put the nose up to slightly gain alt while maintaining my speed.
Now, by the time he got done extending and climbing, he would have been close to 5K away and had an alt advantage. At this point, the decision is up to him if he wants to pursue the fight.
If he did decide to reverse, I would do a long lazy turn back into him, while he was 4K out, to maintain speed and see what he does at the next merge. If he did the same thing, so would I ... keep speed up ... gain a little in altitude.
What you don't want to do is make drastic turns (lose speed) and gain gobs of altitude (thats the D9 domain) ... If he wants to fight me, he is going to have to bring it down to my best alt ... if he doesn't like that, then he will most likely move on to a target that will play his game.
Now, if he continues his lazy attacks and doesn't really pay attention, he will not notice that the La-7 is getting faster on each merge. If the D9, on any pass, does decide to actually press the attack and goes for a guns solution, he will have to slow down ... why does he have to slow down ? ... because I am doing a lazy turning back into him way before he is 2K out and can very easily spoil any gun solution he may come up with if he is going real fast.
So ... if I can get him to "bite" on a couple of successive passes, I have bled his speed and E, all the while I have pretty much maintained mine. At this point, you have to decide when to pounce and if you were patient and have worked him down ... frustrated him ... when you pounce, he will not expect it and he is now yours. Most likely the D9 will nose down perpendicular to the ground, unload for a power dive, and stick stir his arse off hoping that those 20mm shivaks don't find their target.
Mind you, not all Fw pilots fly like that ... NathBDP / Urchin / JB22 are some of the pilots I know that will press the attack so fast, still using the climbing/vert power of the D9 that the La-7 (or whatever) doesn't have time to set up and force the La-7 to turn so often that it will lose speed and E on each pass,and then finally shoot your eyes out as the La-7 is so floppy that it can't get out of the way ... and it will all happen in less than 45 seconds.