I may have found a solution. No Blender model for now, but I can give a qualitative 'frame-by-frame' description of the encounter.
THE ENCOUNTER STARTS
The F-18 is flying west, with a bank of clouds on his left, the upper rim of the clouds is higher than the F-18. The pilot sees on his left, up above the rim of the clouds, a fast-moving light. This is the situation:
The pilot thinks the Gimbal is nearby, just above the clouds near him. He wants to do two things: one is a 'stern conversion' maneuver (as suggested by
@Itsme), meant to bring him to chase the target from the rear, in a favorable firing position:
To do this he needs to bank the aircraft left, to be able to turn left. He also wants to bring his ATFLIR on the target, but the ATFLIR is below the left wing, and he's banking left (the left wing tip is lower than the right one). So he needs to pitch up the nose a lot (possibly to the limits of the flight envelope) to get the target into the ATFLIR LoS, and he surely throttles up the engines. He would also like very much to gain altitude of course, but he's too pitched up, he simply does not have enough thrust from the engines, the best he can do is to keep the same altitude.
The situation is the same as in the first drawing above, but now think the line of sight is the ATFLIR's line of sight.
This is an approximate vertical projection along the AA line marked on the first drawing:
Now the ATFLIR is locked on the target and the F-18 has begun a sharp left turn (while pitched up). The pilot turns on the videorecorder.
THE VIDEO
I only have one frame:
The F-18 is in the middle of the left turn, the Atlas is fast moving eastward. The line of sight of the ATFLIR passes, as before, just over the rim of the clouds. Notice also the angle the ATFLIR does with the airplane axis has decreased.
END OF THE ENCOUNTER
The F-18 has almost completed the stern conversion maneuver (nothing more than a sharp arc of a circle in this case): the pilot expects to find the target about in front (and a bit higher) of him, but it has suddendly vanished. Maybe it was when the Atlas first stage engine turned off, maybe the F-18 just went into the cloud and lost contact, or the Atlas disappeared behind other clouds somewhere along the LoS, this can't be known. In any case the video ends before the target disappears, and approximately at the end of the marked trajectory the Atlas first stage engine turns off (or the stage detaches, I'm not sure about the exact sequence of the first stage separation of an Atlas. However it actually is, at the end of the trajectory the Atlas stops being so a strong IR emitter as it was before). Notice also the angle the ATFLIR does with the axis of the plane is now slightly slanted on the right (6°R, it started from 54°L as we see in the video).
Nothing is seen in the video which suggest the Gimbal is gaining altitude, as the Atlas is instead doing at a fast rate (tens of km during the 34s of the video). This is due in great part to perspective effects (the target is actually very far from the F-18, say 200km just to give a possible figure). How the perspective works is difficult to explain with words, I'd really need the Blender model now, but the basic idea is that the F-18 starts with a high pitch (the nose is pointing steeply up) which then lowers as the encounter proceeds, this even if the Atlas is actually rising up fast in the sky. You can get an idea of how it works by looking to my previous posts with the old Blender model (I don't want to copy it here not to clutter this post with outdated informations): the overall geometry is very different and the F-18 turns with too slow a turn rate there, the clouds would flow the other direction,
@TheCholla was right), however the effect of the perspective is similar, the pitch angle decreased by ~25% in that model, while keeping the Atlas locked on track. Another reason is the unusual geometry with which the video was taken, which makes it hard to figure out istinctively what is moving which direction (see also the next post about why the clouds look to move in reverse), and then the clouds, which are 'fractal' in shape and do not give any firm reference point for our (mine for sure, at least) brain to hold on.
There is yet one important thing to explain: why the clouds in the Gimbal video are seen to move as they do (thanks
@TheCholla for pointing this out to me). It deserves another post though.
----> Continued next