((Here's the thing. And I think this is the crux of my argument, so if you don't mind, I will pick your statement apart.
"Imagination and dreams are even more imperfect and fragmented then memories or reality." Yes, but why are they more imperfect? And even if something is imperfect and fragmented, it can be hard to know it's just your imagination at the time of the said imagination. More on that later.
"It requires conscious effort to maintain them in the case of daydreams" Once again, why does it take conscious effort to maintain them? Because you have stuff to do. Because you feel hunger, or smell something, even if you are not aware of it. Because it's warm or cold, or your ass is hot from sitting in a chair, or because from the sun's position in the sky (or artificial lights) you know it's still day. All these things help you separate the two things. You also know it's a daydream because you've slept the night before and it isn't time to sleep yet.
Now take all of that away. And it no longer takes any effort to maintain them, they come and go as the please. You could make an argument that a workaholic would be able to avoid that because he's always got stuff to do, but I'm just going to ignore that for now. Or perhaps being a workaholic could be a viable solution to the problem. Pity that HMRC are not that.
"or remember them in the case of dreams." Once again, why? Why is it so hard to remember dreams? Skipping all the physiological reasons for why you don't remember dreams, you don't remember dreams because your body knows it is a dream. And what if dreaming and waking is identical to your senses? "And I feel things in my dreams. I can imagine the taste and smell and texture of a perfectly cooked piece of bacon and make my mouth water." Yes, you can feel it with your senses while you are dreaming. And then you stop dreaming, and feel things with your senses in the real world. And there is a transition, an unskippable transition between your dream and you waking up. If you ever dreamt of yourself in your room, you might even have had a dream inside a dream, but you won't confuse it with you actually waking up (at least after a few minutes of being awake). With the robot body, you don't have that transition, so you won't really know when you have stopped dreaming and woke up.
The real horror is when you don't feel anything in the real world. Or, after a while, in a dream. [An interesting derail, but of course your robot body has receptors which you feel. So when I say that you don't feel it is not entirely true. However, since those are not natural to your body and don't work the same way, it is questionable if they are able to replace it or fulfill even remotely the same roles.]
But also besides that, your body knows that it is a dream because you go to bed when you are tired and you wake up when you are not. There are physical and psychological reasons for you to dream, and these physical reasons are an integral part of maintaining a normal (or abnormal) sleep cycle. Take those away, and you rely only on you brain to tell you that it's tired, but without the hundreds of ways for it to express that. (The way your body tells you it's time to go to sleep.) And even if you can reason away what is a dream and what is not, your unconscious might not be able to. I wouldn't be surprised if the logical consequence was you being able to remember dreams as if they were real.
Now, let's get back to this: "Imagination and dreams are even more imperfect and fragmented then memories or reality." That is true, but the real question is if it matters. Having an imperfect dream does not stop you from believing in it at the time of the dream. (Even if you don't, that can change based on the stuff below:) When we view our memories, we add and remove and rearrange them as we see fit. That's not a secret, it is a well-known fact. Now if take all the things I have said before, the body no longer having any indication of what is a dream and what is not. The lack of transitions, the lack of physical indicators. The brain treating dreams as reality.
You end up having dreams that are indistinguishable from reality, lack a regular cycle, have no transitions between entering and exiting them, and whenever you remember them, your brain will do it's best to add and fill in all the details. It is here that the whole thing about being mentally unstable comes into play the most. And I do not know to what extend it will happen, and it is quite possible that how it happens will depend on the state of your health. But I hope you will now at least understand why I keep saying that it may be impossible to distinguish between the two.))