Ok, called in a plumber this morning, and the problem has now been solved, and we have hot water again. The problem was a plug, but not as we expected...

http://i.imgur.com/PFm4VI0.jpg

This image is what remains of the original cold water dip tube, plus another little surprise.

The original dip tube is broken off, obviously trapped inside the tank. That explains why I had such a small amount of hot water - cold was running straight up the hot water line, they being so close together with no dip tube to bring cold water to the bottom of the tank. So that's the first problem solved. The plug, though, was caused by this little white plastic marble that we found by doing another, longer test at the hot water out nipple. It turns out the first heat trap marble that I saw was fine, but this second marble that somehow got into the tank was getting sucked into the hot water out nipple from below. This explains why when I did my first test there, I thought water was flowing through it: I didn't allow water out of the hot water out nipple long enough for this marble to be sucked back into the nipple. Which of course I never could have known.

We suspect that there must have been some kind of pressure trap in the original dip tube, and when the dip tube broke off it let loose this marble into the tank. It floated at the top of my tank from then on, and couldn't have been sucked into the hot water out nipple until I had drained the tank while cleaning it. Then when I refilled the tank, it became stuck there any time there was enough prolonged water pressure to suck it up.

So, I now have hot water again and took the longest hot shower I've ever taken in this house.

Also of note, my plumber informed me that my piping is Poly B, which is big time bad news. He informed me all about it and left me an information package on it. Luckily we don't plan to keep the house for forever.

Thanks everyone for your efforts again, I really appreciate all of the help you all provided me with. This was definitely the best plumbing help site I found on the internet.

And we thank you for the update. Glad you're back in business, Tom

Oh well, guess I just wasn't cut out to be a plumber.

We got snow today Mark, but not enough send you a snowball. How about a pizza instead.

That's cause you were cut out to be MUCH more than a plumber, Harold! This is just a fun hobby for you nowadays! Nice job on this one... really stuck by Loren!

Turns out more snow is headed my way overnight! UGH! I've had about enough his year!

Plenty of snow here, AGAIN!! So you guys get down here and take as much as you want (hopefully all of it). I just figured I would post one last time (or first time on this particular series of posts') to make this possibly the longest running water heater issue to date. Good job guys, and glad you got it taken care of, loren.

Yeah heck, the dedication you guys showed to the problem was admirable! Anyone could have shrugged a long time ago and said "welp, can't help you." You should be commended for that alone :) Besides, this seems to be a one-in-a-million issue, and to have to try to solve it over the internet with nothing to inform you but an under-educated user, you can't really be blamed. Hehe. Thanks again everyone.