Is this where a 51% attack could make this plausible? 1 person broadcasting a block to the network would immediately be rejected but if 5100/10000(for example) broadcast the same fabricated block, this could fool the system, correct?
a 51% attack is actually when 51% of the mining hashrate/hashpower somehow attacks the network, the most common fear would be to execute a double spend attack.
All that would happen if someone did what the OP proposed is would create a premined altcoin.