First I gotta say that that is a beautiful photograph, the egret in the foreground really sets it off.
Well you sound as though you have the right ideas about restoring her upper works.
As for the bottom planks, no you don’t have to replace a full plank.
However, check them thoroughly, those worms can travel for bloody miles along the grain of the wood.
They can grow up to half a meter long and survive for weeks even with their burrows sealed.
Hopefully the damage, as you say, is limited to small areas, then that is all that needs replacing.
The ideal way to do that would be to scarf the replacement timber to the existing plank.
Te problem there is that without removing the original plank it is difficult to cut the scarf join accurately.
The next best thing is to butt joint the replacement timber to the old plank.
As long as you are using the same type of timber or similar as the replacement and it is only glued to the existing plank and not to the frames, then there should be no problem when it comes to taking up.
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