1.The answer to your first question is "Everything begins with duality and ends with non-duality". Therefore, when a soul is formed as a part of Infinite consciousness, it is dual i.e. the part is not the same as the whole. But when it attains mukti, it ceases to identify itself as a soul and is perfectly tuned to the Supreme will and consciousness and is therefore the infinite.
2.If you can see, I have written that the Infinite does not have any particular form. This means that everything and anything is a part of the Infinite, even if it is formless or has a form. Be it anything: everybody, materials, plants, planets, cosmos, you, me, anybody and space between anything, all are a part of the Infinite.
3. As for matter, though it does not have a consciousness of its own(i.e it does not possess ego), the Supreme Consciousness pervades it.