If you do not configure the pins, they are initialized as pinMode(xx,INPUT) which is a floating high impedance state. Per the Arduino Uno R3 schematic, that pin is on PB5/SCK/13 which is run through the op-amp (top center in the schematic) to drive the LED. Per the Arduino Mega schematic, that pin is on PB7/OC0A/OC1C/13 which isn't run through the op amp. ...
Presumably you will avoid having two Nanos try the same item from the
list. Then, only one Nano can find the answer, and only one will try to
transmit data at any given time. In such a situation, yes, you could
connect all five Nanos to the same serial link:
Connect the Uno's TX directly to the RX each Nano. They will all
receive the data transmitted by the ...