1. Call ESP8266WiFi.Connect to connect to an existing wifi network.
2. It is currently not possible.
3. Not sure that I understand the question. WiFiSocket and WiFiServerSocket are exactly the same as EthernetSocket and EthernetServerSocket.
1. I know how calling connect to ESP8266 STA.. I need switch module to STA only (STA only=6mA standby. This mode AP+STA have 80mA standby). Command in C is WiFi.mode(WIFI_STA);