You could validate block difficulty, but how would you validate someone's mempool size? But since you said it doesn't need to be validate it, what would happen if majority miner claim have big mempool or majority miner claim have small mempool?
In addition there is no financial incentive for miners to attack blockchain.
Regarding maximum block size, there are possible financial reason to do it such as,
1. Lower block size could force people to pay more transaction fee.
2. Higher block size could be used to attract investor with PR "Bitcoin now is scalable and can adjust to network condition" which increase Bitcoin price.
1. Lower max block size can have minimum size set to the current Bitcoin block max size so it cannot get worse than what it is now.
2. If it does increase Bitcoin price than I would not complain
. If it does not then the incentive to claim big mempool for miners is gone.In real life different miners would set mempool size to different values, there will be conflicting interests, and in the long run it will be close to what majority of nodes see.
But I am not in favor of increasing max block size as I believe it would affect decentralization. But theoretically it is an interesting topic.