Hi, James...
1. No. Neither machine will ever be Net connected when the link between machines is used.
2. Both are connected to the Net, when I'm at them, using only a broadband dongle - this home has no fixed broadband service.
3. 21 feet (deliberate overhead allowed for) and all obstructions can be drilled through.
4. This home has no Ethernet outlets, at all.
5. I have no modem or router...there is no fixed internet service. I use only dongle broadband.
I literally want this to be wired, only, and I've no idea as to whether or not that can even be done. I live alone and the landlord has already said that it's okay. I just need to go out of this upstairs machine, through the floor, along a picture rail and then just drop the cable to the floor, from where it'll be connected to the rear of the downstairs machine. I have concerns about the strength of the signal/s because of the length of the required cable, but I know too little about this subject to be able to judge on that, at all.
Thanks for your assistance, so far.