The spark soul has certainly form which means hands, feet, etc. This we learn from Bhagavad-gita. The body is described there as Vasamsi, which means dress. So unless one has got originally hands and legs, how the dress, coat and pants and shirt, takes such form? Therefore the spirit soul has original form. When he is in the material energy the dress is evolved materially and when he is in the spiritual energy, the dress is evolved spiritually. This is also not very difficult to understand, as our students before coming to my contact, he was supposed to be materially dressed, attached to sense gratification, and after devoting himself in Krishna Consciousness, he is gradually developing a spiritual dress. That means attached to satisfying the senses of the Supreme.
Your second question, how the impersonalists mistake the difference between the soul and the Supersoul—The mistake is due to their obstinacy. The impersonalists masquerade as Vedantists, but actually they are defying Vedanta. In the Vedanta it is clearly said, the Original Source of all being; in the Upanisads it is clearly said that the Supreme is the Supreme being of all living beings. So all the Vedas affirm it vehemently that the Supersoul and the soul are two different identities, although qualitatively one. But the impersonalists they accept Vedas as authority, but they go against the verdict of the Vedas. Lord Caitanya has depicted this impersonalist class of men as more dangerous than the Buddhists. The Buddhists plainly declare that they do not accept the authority of the Vedas, but the impersonalists masquerade themselves as followers of Vedas, but actually they are hidden Buddhists. The idea is, if a person is actually fast asleep, it is easier to awake him but if a person pretends to be sleeping but actually is awake, then it is very difficult to wake him up. So from all Sastric point of view, the living entity and the Supreme Lord, or the Supreme Living Entity are always simultaneously different and one. One in quality, and different in quantity. this simple thing is understandable by any common man, but these impersonalists, they will simply invent jugglery of words to mislead innocent people. Therefore Lord Caitanya has warned not to associate with these impersonalist mayavadis because they will spoil one's life by diverting one from devotional service.
It is very difficult to bring to reason the obstinate impersonalists. For example, Prahlada Maharaja, such a great authority, could not convert his father to devotional service, who preferred death, and still did not agree to accept that God is different from him. So better to avoid the impersonalists as far as possible.