Will this work elegoo mega 2560?

Yes it fits, you can remove the green encasing over it for it to be even smaller.

Does this mount give access to the tx/rx pins (Serial1-3)? I suppose I could convert my code to software serial but i'd prefer not to.

Yes, and you have access to the other serial pins as well.

Can I use this module with an Arduino Mega with the ethernet shield attached?

The module just converts the existing Arduino Mega I/O pins to screw terminal breakout. As long as you stack the shield on top of the Arduino Mega, it should be no problem. Never used an Ethernet shield with a mega though.

ASIN	B01CU3OB5Y
Best Sellers Rank	#221,310 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #134 in Terminal Blocks
Item model number	6520
Manufacturer	Electronics-Salon
Product Dimensions	8.7 x 8.7 x 4.5 cm; 220 g

Electronics-Salon DIN Rail Mount Screw Terminal Block Adapter Module, For Arduino MEGA-2560 R3.

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