Every forger creates a very large random number. He hashes this number and adds the hash to to block he is forging.
How will everyone else verify that the number was indeed generated randomly? The forger could have used a biased generator, or just chosen it themselves.
Yes, but before the number will be used to seed the random function, it will be hashed together with 100 other numbers of other block forgers. If at least one of these 100 numbers is random then the whole resulting hash is random