Ethereum Mainnet congestion affects how long a transaction takes to confirm, but it's historically had little bearing on ETH's price itself.
A card payment has no price history to check — the fiat price is fixed; ETH's Stars-equivalent price moves with the market instead.
Choosing a different Ethereum Mainnet doesn't change ETH's price — it only changes the transaction fee paid to send it.
It covers recent trading history, giving a general sense of how ETH has moved rather than a full historical archive.
Yes — ETH's price is the same regardless of which network it's sent on; only the transaction fee changes with Ethereum Mainnet.