GJ Newby here. I've been following this thread for a month or two now. Gotta say you've inspired me to get rid of much of the junk that I've been toting around from house to house....just donated a bunch of decent stuff to a charity yard sale this week. Just wanted to say good luck Bill and...
No if you want to run a cat-5 cable it will need to be plugged into the router, not spliced in the attic. You could probably do a little rewiring and move your router into the attic and feed your office in the house and your 200' run from there.
Good Luck,
Mike.