Q3: you could use a pot to control the dimming. It would not/cannot be connected directly to the strip. You would connect it to an analog input on the arduino. Your code would read the pot and adjust the brightness using a function of the FASTLED library.
Q4: think we covered that now?
Q5: think you got Tx & Rx mixed up in the question. I don't see the diode you mentioned, unless you meant the led? The Uno will be receiving the data immediately, it is the code running on the Uno that is not dealing with that data for 1~2 seconds, due to mySerial.readString() timing out. It is waiting for an end-of-line character, but the esp is not sending any. Try changing Serial.print('B') to Serial.println('B') in the esp code.