If your node changes a block, the hash of that block changes and probably the state root.
A few things happen.
Every block after is invalid. You would need to remine them all.
The blocks your node receives from others would appear invalid and blocks your node sends would appear invalid to them. It's a fork unless all nodes agree.
The threshold for ...
The gas limit can change overtime based on the targetgaslimit set by miners (this is geth terminology). The gas limit is capped with an upper and lower bound and can only be changed so much with each new block. So the gas limit can be dynamic, but generally there really is only gas limit at any point in time.
Often times blocks do not use all of the gas ...
Vitalik Buterin gave an example of using the different trees (transactions, receipt, state trees).
Has this transaction been included in a particular block?
Tell me all instances of an event of type X (eg. a crowdfunding contract reaching its goal) emitted by this address in the past 30 days
PoA networks do not natively have rewards baked into the consensus. There are instances where these are created with smart contracts that make system calls (see Energy Web Chain and POS_DAO) , but if you run clique natively there will be no block rewards.
Energy Web Chain - System Contracts - Rewards Contract