Indeed, Engels is clear that he takes matter and motion to be an abstraction from what we sensuously perceive, and so unless one wants to conclude Engels is an idealist, one has to necessarily conclude Engels is a direct realist. He is clear that matter only feels like it cannot be perceived directly because when we are talking about "matter as such" we are talking about a metaphysical abstraction of matter, but if we are more concrete and specify the context of the matter we are talking about, we can indeed directly sense it.

The two forms of existence of matter are naturally nothing without matter, empty concepts, abstractions which exist only in our minds. But, of course, we are supposed not to know what matter and motion are! Of course not, for matter as such and motion as such have not yet been seen or otherwise experienced by anyone, only the various existing material things and forms of motions. Matter is nothing but the totality of material things from which this concept is abstracted and motion as such nothing but the totality of all sensuously perceptible forms of motion; words like matter and motion are nothing but abbreviations in which we comprehend many different sensuous perceptible things according to their common properties.

Hence matter and motion can be known in no other way than by investigation of the separate material things and forms of motion, and by knowing these, we also pro tanto know matter and motion as such. Consequently, in saying that we do not know what time, space, matter, motion, cause and effect are, Nägeli merely says that first of all we make abstractions of the real world through our minds, and then can not know these self-made abstractions because they are creations of thought and not sensuous objects, while all knowing is sensuous measurement! This is just like the difficulty mentioned by Hegel; we can eat cherries and plums, but not fruit, because no one has so far eaten fruit as such.

Engels is clear throughout the book he is tying his understanding of material reality to observation. There is one section where he criticizes the towers of abstractions built by mathematicians and insists upon them coming back to the real world and see how their mathematics are actually used in practice, and that is the reality of the mathematics, its connection to the world in real-world applications, a very Wittgensteinian approach.

i think i now better understand your point, but you are getting confused a bit. engels is not a direct realist he is a diamatist. yes, engels declares matter and motion as ideas, but your confusion comes from an idealist understanding of ideas themselves. for marxists the human observes the world directly and then creates ideas, but this ideas are not nuomenal, they are just another representation of matter in the form of thougth. it would be something like this. we perceive motion, we create the idea of motion, then we take into practice (with the material world) this idea of motion to deepen our understanding of it.

another way of looking into this is like this:

[ a = \frac{\Delta v}{\Delta t} ]

this is what engels would call "motion is nothing but the totality of all sensuously perceptible forms of motion" and it is not until we use a specific case that the variables can be filled. but the fact that it is an idea does not make it any less material.

i wouldnt call that statement wittgenstenian, it is diamat. look what mao has to say:

Discover the truth through practice, and again through practice verify and develop the truth. Start from perceptual knowledge and actively develop it into rational knowledge; then start from rational knowledge and actively guide revolutionary practice to change both the subjective and the objective world. Practice, knowledge, again practice, and again knowledge. This form repeats itself in endless cycles, and with each cycle the content of practice and knowledge rises to a higher level. Such is the whole of the dialectical-materialist theory of knowledge, and such is the dialectical-materialist theory of the unity of knowing and doing.

as for the rest i am unsure i can say much sincw i know very little of physics but from what you say it sounds like bogus