I disagree, no one will have Clams invalidated and things will remain as they are for Clam holders.
This is wrong in general. You forget about CLAM holders who bought today for example. Those will be screwed.
You are assuming a retroactive self-serving block is chosen, which it may be. In that case, yes, those who bought or dug after the block would be disadvantaged. I'm guessing whoever is choosing the block would not be among them (at least not to a significant degree).
If the block to be chosen is assigned is in the future as I recommend, then everyone can decide whether to buy (or sell or dig) before or after the split.