Looks like you may be interested in software called "Tabletop Simulator". This will allow you to create a (or download someone else's) chess board. Here are the features that you're interested in:
You can do anything you want to the pieces and the board
Doesn't check for any moves (doesn't process anything you do)
Pretty awesome UI - I believe you can hide/...
As per my answer to this question (sometimes it's better to search before asking ;-)
You appear to be seeking Toledo Nanochess.
With only 1257 non-blank characters, Toledo Nanochess is the current world's smallest chess program in C language.
Doing it over serial port is left as an exercise for the reader :-)
The chess.com site allows "take back", if your opponent agrees, in unrated games only & they have an iPhone app for both free and premium members but I have to say that I have not tried it as I do not have an iPhone so do not know for sure if that feature is available in the app.
There are a lot of chess engines written in C#. Unfortunately, 99% of them use the GPL.
All except this one. It uses the LGPL, but unfortunately, its very early in development, so you might have to choose using the GPL, or using a very early engine.
I've been looking for the same here. I haven't found a LGPL chess engine in C++ / C#, but if you can accept Java as the programming language, you can take a look at FluxChess http://fluxchess.com/flux/