If the primary anode is gone, you can presume the secondary one is, too. As to getting it out, it doesn’t look as bad as some I’ve seen. There is a space between the threads to put a wrench on that won’t wreck the threads.
One of my mentor’s tricks is to drop a bolt into the nipple that is nearly the same diameter. That keeps the nipple from deforming or collapsing if it is in tight.
If the primary anode is GONE, you have a few choices. I’d do the external inspection described in the Know-How entry under The Basics. If that looks OK, you can either drop a powered anode into the tank or put in a .90-inch-diameter hex (providing you have the head room; otherwise the flex version, which is also .90-inch, but doesn’t have as much sacrificial metal due to the links), wait a year and see what happens.
If the tank hasn’t broken, you can send us a photo of the anode and we’ll tell you what to expect. I’d kind of lean toward doing that over the powered, as after you’ve stretched out the powered anode electrode, it’s pretty much only good for a tank of the same height or taller.
But I’m thinking that if the water heater survives a year with a new sacrificial anode in it, you could switch to a powered anode and have some confidence that it will go indefinitely.
Feel free to ask more questions.