Blockchain Statistics – How to Find Transactions Per Second Data

Ether between testnet and mainnet can't be transferred: they are separate networks.

But you need to be aware and careful of replay attacks if an attacker can see what you're doing on the testnet and has the private keys to the accounts being used.

Also, you can spend the testnet Ether you received (at 0xc2a2c4...) since you already have the private key for it: private keys don't know anything about the network.

There is some info and calculus here What is the size (bytes) of a simple Ethereum transaction versus a Bitcoin transaction? that there is around 75 trx per block. I found some other that says there is 150 trx per block. So i guess it is somewhere in that space.

About the block size, there is also info in link above it is 15kb per block.