You have an issue here, I grant you that.
It sounds to me like you may indeed have a mobo issue going on. My stock answer to this problem is to call Dell and ask for a replacement. Unfortunately, Dell is horrible with customer service, especially if the solution costs them any money. I think you may be stuck with fighting them to replace it, which is going to be frustrating, as I can attest from past experience.
To answer your second question, I have never installed anything but a Dell mobo in a Dell rig. Can you put another brand in it? I honestly don't know, all I have to refer to here is my Dell Inspiron, which is a laptop. I would think the first step is to look at the chassis in your rig and see if it will accomadate a nomal ATX mobo. If you have an extra mobo in the house, see if it will fit in the tower.
As for your last question, you may get an error when the rig boots up saying, "Non system disk" or it may just boot right up. I've had both scenarios happen to me. At the least, you're going to have to install drivers for the various components on the new mobo like the ethernet adapter, usb hub and onboard sound if that's what you get.
I think your best bet is to try and get Dell to ship you a new mobo, just be persistant with them. You might also try and find a used one on Ebay, I've done that with HP rigs before and had success with it.
If you do manage to get another brand of board to install and work, let us know how you did it.
I\'m already visualizing the duct tape over your mouth...