Fraid when it comes to not being able to clean, it is immaterial whether the cat will allow a diet or not, it has to be done - we are the humans, and we have to do these things for them, as otherwise, it is verging on cruelty, as the weight will only continue to get worse, and trust me, it is a lot easier to monitor while they only have half a kilo to lose than get to the stage where they have 4 kilo's to lose. There are also the health issues such as diabetes, liver issues, arthritis etc to consider - how would you feel if your cat ended up with a lifelong health issue because you didn't want to be cruel to them? Exercise also has to be done in conjunction, as with humans, no point only doing half of it. It can be very hard, I have had 6 cats in the past with some needing to put weight on and some needing to lose weight, but it can be done.