How i can be sure that private keys not stored on centralized servers?
In addition to what the user above said, the servers that Electrum connects to are not made by the developer of the wallet (ThomasV), they are made by people from the bitcoin community so It wouldn't make sense If the developer will make the private keys sent to somebody else, those servers only know your addresses and nothing else.
When users enters private key it must to be send to bitcoin blockchain servers in encrypted format, how i can be sure that no one can see them, use packet sniffing, etc..?