[cesarb]

> My impression is if anyone has 51% of all bitcoin they can write whatever they’d like on the blockchain and validate it at will. Is this wrong?

Completely wrong. There is a set of rules which define whether a Bitcoin block is valid or not, things like "the block's hash has this many zero bits" (simplifying a bit here, the actual rule is a bit more complex), "all transactions have a valid signature" (simplifying a lot here, it actually runs a sort of a small program) and "no transaction spends outputs which have already been spent". It doesn't matter how much Bitcoin you have or how much hash power you have, even if you have 100% of all Bitcoin and 100% of all hash power, if the block doesn't follow the rules it will be ignored by all validating Bitcoin nodes.

What having 51% or more of the Bitcoin hash power gains you is only that you can rewrite history. You can present a chain of blocks to other nodes, and later present a different "longer" chain of blocks to the same nodes, and they'll accept the "longer" chain and discard the older one. All these blocks, however, still have to be valid to be accepted.