All miners produce diff 1 shares at the same speed relative to hashrate (let's ignore the variance there). They do this no matter what share difficulty is wanted by the pool. Every one of those shares has 1/pool_diff chance of being accepted by the pool. Saying "it would've taken 31 seconds to finally get that share, but because the block took only 30 seconds I never had a chance" is just blatantly wrong.
That doesn't make any sense. The whole point of a 512 diff rate is you send data to the server 1/512th of the time vs 1 diff. If they never get sent, how are they going to be accepted.
It's simple. The miner generates diff 1 shares. If a pool requests diff 512, the miner checks every share against that requirement. The miner ends up sending on average 512 times less shares, but every share is checked locally by the miner, and so has a chance. There is no magical 'buildup' to diff 512 that takes a random amount of time.
Its 512 times harder to solve, therefore it happens 512 times less frequently. During that longer period of time, if a block is found, your work isn't recognized. That's all shares do, prove you did work.