I'm a little confused by what you are trying to do. You don't make it clear whether you are trying to extend an existing wireless network to include devices on your ethernet device or whether you are trying to create a new subnet and use NAT to hide this from the wireless router. The good news is that both are possible and are fairly simple.
Personally if you are just wanting to temporarily make a few devices accessible via the wireless router I would tend to go via the bridge route. That way your existing wireless router will handle IP address assignment etc. The instructions for setting up a new subnet are a little more involved because you will likely need to setup a dhcp server on your laptop too.
Answering your 2nd question you can do most of this direct from the commandline on an adhoc basis. That way if you mess things up a simple reboot would restore your original settings.
To use the bridge approach you need to install the bridge-utils package on Debian
apt-get install bridge-utils
Note you might need to prefix the line with sudo if you do not have a terminal window open as the root user.
Once you have bridge-utils installed then look at /usr/share/doc/bridge-utils/HOWTO and the other documents in that directory they give you a good explanation of how to set things up.
Note one issue you may have to workaround is if you are using network manager to manage your networks on the laptop. That can tend to override what you do on the commandline you might need to temporarily stop it managing the wireless and wired interfaces on your laptop whilst you setup the bridge.