Hello Mocha, and welcome.

It sounds like a combination of things has changed your dogs behaviour and I think you have a pretty good angle on what is going on. His age may be a factor, although I suspect that there were signs before now that your dog was pretty dominant?
The new kitten is a certain factor - as is the way we tend to treat cats and dogs differently. Cats do not tend to have the same restrictions that dogs have and to a dog that feels it is king of the heap this can be a trigger for it to assert it's dominance more strongly.

I wonder what you mean when you say the dog is, "more aggressive"?

Get a muzzle or get your vet to show you how to put on a safe tape muzzle and continue to bathe the dog - ignore all protests.
You need to get control here and make it clear that you are the boss - not the dog.
Have a look through the other behaviour problems on this site and you will find a lot of suggestions that will help you - eating problems and separation anxiety posts will probably have the most useful to you.

Hope this helps - if not - just ask again!