Say I have a sequence of 2000 images, and I have 11 keyframes and thus ten blocks of 200 images.

After I have processed the whole thing and I view the movie I see a small problem with the central keyframe, which I then redevelop in LR.

Question:

Can I save metadata just for the central keyframe, reload and retransition the sequence in LRT which will obviously affect just two blocks of 200 images?
Can I then reload just the 400 frames metadata in LR?
Can I then export and overwrite just 400 intermediate images before rendering the movie again, and
Do I then need to render the entire sequence ?

I use arbitary numbers of images in this example, but small adjustments to long sequences can take a long time to process.

I have experimented with this already but always end in a mess !

Yes, just reedit the one keyframe, then proceed from the 2nd workflow row in LRT. Easiest would be to reexport the whole sequence, but you could also select the range to reexport in Lightroom, and then set the first sequence number in the export dialog to the number of the first frame that you reexport (advanced settings in LRTExport panel).
So if you want to reexport 200-400, select those frames and set the first sequence number to 200 in the Export Dialog.

Superb ! Thanks Gunther , resetting the first sequence number is what escaped me; I get it now.