Dogs will often attack a dog that has health issues or to help protect the owner. Dogs also will attack a lower ranking member of the pack if they do something that is not acceptable to the alpha. The alpha position may be changing in the household as well which can cause one dog to correct another's behavior . It could be a new health issue in Russel that Cosmo is reacting to. Cosmo may also be having behavioral changes as the result of the seizures and the seizures might be linked to a health concern. Hypoglycemia (low blood sugar) can cause seizures and behavior changes are often present. Since you feed them a meal at night, that doesn't seem as likely. However, if the liver is not functioning properly, feeding might actually cause some toxins to build up in the body and lead to seizures, so it might be a good idea to get cosmo a full checkup. However, he may just be reacting to Russel. I just wanted to touch on this in case Cosmo has issues as well.
Russel being deaf may rely on his other senses more and it may be that his sight is starting to go as well. This might lead to him getting up in the dark and not being able to see as well as he does during the day and explain his barking and growling. He might also be starting with a condition called Cognitive Dysfunction Syndrome (CDS). Here is a wonderful site that explains it well and it has been seen in dogs younger than yours. There is a medication that can help with this problem, so discuss it with your vet. Please see this site for more information.
Dogs with CDS often display different behavior such as not recognizing people they should, getting night mixed up with day, asking to go out and then wanting right back in, and other abnormal behavior. It might also be something about the matress itself that is causing Russel to wake up. Most people sleep longer at night then during the day, so it may only happen at night because the dogs have been asleep longer.
Cosmo may also have decided that since the matress is new, no dog owns this mattress while Russel may have "owned" the old mattress being alpha. Now that Russel has lost his hearing and may not be the alpha, Cosmo may be claiming the mattress and telling Russel to stay off his mattress. The Russel wakes you up and since you are the ultimate boss can then jump up.
I'd start with checking them out medically first to be sure it is behavioral. If it is, you might want to start some sort of obedience training with both of them, so they see you as the ultimate boss. You might also not allow either on the bed and give them each their own dog bed off the floor, so there won't be an issue with your bed. It doesn't have to be permanent, just until this behavior is under control. You can then make them work for the privilege of getting on the bed.
The following site is helpful for teaching you how to train your dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
You can also leash Russel to you (be sure it is on a harness and not a neck collar) which might help prevent him getting down and stop Cosmo from attacking when he jumps back up. You can also leave a leash on cosmo (not attached to you) and if he tries to stop russel from getting back up, you can give him a short tug to get his attention and a firm "NO" to indicate the behavior is not acceptable to you and you will not allow it. It may even be Cosmo who is waking and making Russel get down off the bed. Unfortunately, unless you have a video camera to tape the behavior when you are sleeping, that would be hard to find out if it is happening.
If you have a mattress cover that you used with the old mattress, that might help with the situation. It would be worth a try and might help. I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .