Both dishwasher and washing machine might be good choices. These things work hard and take care of business, but they have no intelligece beyond what they have been programmed to do. They do all the work, but the thinking is coming from an entirely different source.
Going back to #4 above:
we are looking for something that works very hard and strives to do better, but will ultimately be replaced.
I'd choose a handheld grater. It's one of the most primitive kitchen tools. Years ago they were simply made by poking holes in metal surfaces with a nail. Now they are often replaced by electric gadgets, foodprocessors and the like. They are also hard work. (Ever try grating cups of shredded carrot?) ;) And I sometimes feel that I lack intelligence when using one (and grate my own knuckle!)
What about a nail? Boxer is in some ways what holds the farm animals together like a nail holds two boards together, but he keeps getting "hammered" by the leaders. They pound him with more and more work, and he keeps doing his job. A nail, when it rusts, is useless and discarded, unlike a hammer which is just used on another nail.
That is an interesting project! How about a door knob. I am not sure if you are familiar with this figure of speech, but when I was a kid people sometimes said, "You dumb as a doorknob," when they really wanted to underline a person's lack of intelligence.
Another suggestion is a closed book. This might have a paradoxical message. A closed book represents foolishness and the lack of learning, but an opened book represents just the opposite.
Finally, how about a TV. Mass media is, after all, brainless and requires little critical thinking.
I think that if the task is to find a household appliance that represents Boxer, we have to outline some of its essential characteristics. Boxer's tireless work ethic, and his belief in having to "work harder" is probably the most critical element about him. It's not that he is not intelligent, but he genuinely believes that working harder will allow for social advancement. Hence, we are looking for something that works very hard and strives to do better, but will ultimately be replaced. Perhaps, a kitchen appliance might be a good bet for these conditions. A dishwasher is designed to do much work, continue to strive in its continually repetitive actions, and, in the end, it will die due to exhaustion, only to be replaced by another one which will promise to do the same work in "twice the time."
I would go for a sponge. A sponge could be said to represent a lack of intelligence in that it is empty of anything until it begins to pick up liquid (knowledge?) like a brain. Also, if I remember rightly - sponges are a very low life form! Natural sponges even look a bit like brains in their shape and are full of holes and vacuums which make them absorbent. This could represent space waiting to be filled with facts,knowledge,information or intelligence. An empty sponge can also be used by other people for their own ends - ie manipulated to suit the motives of the user. This might suit the theme of politics/power where educated and controlling political activists use the less educated proletariat/peasants to drive forward their policies or do their dirty work in heckling and propaganda.
A bar of soap might be a good representation of lack of intelligence. One is not able to engage in a conversation with it. The soap has one primary purpose to be used to clean with it. When it becomes used up the remainder of it is thrown away. It has no significant meaning or depth to it. It is what it is and nothing more. It has no way of growing and further developing.