I have a very tight space for a project that requires a mega board , the "power in" plug takes it over the space available. Are there safety or other concerns soldering to the lugs for the "power in" socket on the reverse of a mega 2560 rev 3 board board? the board is fitted with an elctrofreaks TFT LCD MEGA SHIELD v2.2 and a TFT_320QVT LCD screen. (the elctrofreaks mega shield uses vin pin) 1 other PWM pin is used and an esP8266 board on a uart channel The power is from a regulated power supply. If I do this, am I correct that the regulator and polyfuse will still be used?
Soldering to the underside of the connector is exactly the same as plugging something in to the connector.
Yes the regulator will be used.
No the polyfuse will not be used - that is in the USB power circuit not the barrel jack power circuit.