The others are all answered well except for this. Generally, it is okay for someone to assume that the address corresponds to the one that you control. That is, assuming nothing is actively changing the address you're seeing and/or your clipboard. I would assume the worst and attempt to cross-check the address on my cold wallet as well, just a simple validation to check if I'm really sending to the correct address.

The purpose of your online wallet is to check for balance as well as to create a transaction. There is no protection if you've downloaded a malicious version on your online computer or if a malware can change the addresses in your online Electrum instance.