Sounds like they are doing what a lot of other companies are doing these days, they have started publishing a number 20% under the actual rating of the tire because of some litigation that happened a few years ago over trailer tires, they were putting the actually carry capacity on the tire and people were overloading them and blowing tires at speed, so they started publishing under rating by about 20% to compensate for what they know owners of them will do. Overloading trailers is a very common thing now a days, It also depends on the axles, yours should have Dexter torsion axles in it, which are also under rated a bit to compensate for what they know is going to happen in the real world.
Now, as far as ordering online and having them mounted locally, you need to check with Maxxis to see if they have a list of tire stores in your area that can mount them and service the warranty, not all tire companies use what the manufactures think they should and then won't honor a warranty if something happens with one of the tires. So just make sure that you can get service and after the sale problems taken care of, if needed. Ran into that a lot when I was helping my friend out with his shop.
By the way, how many miles do the tires you currently have on your trailer have on them? and are the tires showing excessive wear?
As far as being discontinued, it could be as simple as Maxxi does not want that company selling their tires any longer, I have seen that happen before and seen discontinued notices on websites that were two years early only to find out they did something to make the company mad so they pulled the line from them. Maxxis could have sent a notice that they were going to discontinue the tire sometime in the future and there is still a lot of stock out there, or they could have simply changed some of the information that is imprinted on the side of the tire and they decided to change the model number. You just never know.