Have you upgraded to 8? Or do you still have 7? The program to go beyond the 4 tuner limit is called "Tuner Salad" and it runs on Windows 7 at the moment. Run that, it should let you run a lot more recordings at once. If you do run a Ceton card, the drivers for it automatically raise the tuner limit for you. I just did a quick google on it and got back a few results, so if you went that route, I think you would be alright. Looks like the SiliconDust drivers are supposed to do the same thing. I only have the 3 tuner box, so I don't know about all that. (Btw, the other one is a 6 tuner box, not a 5. Might have just been a typo, just thought I'd mention that.)
When dealing with the local office, it doesn't surprise me at all that they screwed up your account. I have had a lot of problems with them lately. I've barely had 1 month go by without having some sort of billing issue with them. I have suggested they have a complete office wide retraining of every employee because their employees don't seem to know what the hell they are doing. I think I am finally squared away, but not after getting a complete credit for one whole month of service.
Now, as far as taking the M Card out of your second TiVo, why would you do that? You already have service on it, might as well keep it going. You can just pick up another card from the office for free. The first card in each device is always free. It is the second that you pay the $2.50 for. Good luck in getting a card that works, though. From the Shackleford office experiences I have had, it is a crap shoot. First, they don't have cards that are ready to go like they are supposed to at the front desk, so they have to go in the warehouse to get them. Then you have to worry about it working at all. I still have a card sitting here on my table that will get a signal from them, it just can't communicate back to their junk. I've tried it in two devices and both times got them telling me that they couldn't talk to it. I sent back a, probably, perfectly good TiVo Premier box because of the crap. I am kinda pissed about it. Anyway, yeah, I suppose you could take it out of the TiVo and tell them to re-pair it to the HDHomeRun Prime that you get and it should work. Shouldn't mess up the billing at all since you get the card itself for free. Just make sure you have @ComcastWill do it for you. He seems to be the most competent.