Critique of Pure Reason:
Kant made a difference between 1) sensible intuition and 2) non-sensible intuition. Here intuition translates the German Anschauung. Kant always uses this term for our mode of processing our sensible input. The output of this first step is structured by space and time.
Kant names the source of our sensible input thing in itself. Kant’s point: We cannot conceive the thing in itself, we cannot conceive any property of the thing-in-itself, we cannot apply any category to the thing-in-itself. The thing- in-itself is a pure hypothesis, necessary as a base to the whole process of cognition. Hence noumenon in the negative sense is the term which fits our manner of cognition.
On the opposite, noumenon in the positive sense would refer to an intuition different from human intuition with time and space. Kant states that we are not acquainted to such type of intuition and that we do not even know whether it is possible.
The whole passage from the Critique of Pure Reason does not relate to the difference between potential infinity and actual infinity.
Potential versus actual infinity:
The standard example to illustrate the difference are the natural numbers 0,1,2,…. You can continue counting without reaching a last number; that’s potential infinity. On the other hand – at least since Cantor’s set theory - you can consider the set of all natural numbers. This set is actual infinite. Aristotle accepted potential infinity but not actual infinity.
To your first question: Paraphrasing the last statement by using your terms I mean that Aristotle’s idea of potential infinity does not contain the idea of completion.
To your second question: What do you mean here with “in itself”? Does it relate to Kant’s term “thing in itself”? In any case, potential infinity and actual infinity are two different terms. Neither contains the other.
To your third question: According to Kant we cannot know anything about a noumenon, notably we cannot know whether it is infinite in any conceivable sense. We cannot even know which concepts apply to a noumenon.
In general, I consider it difficult to speak in abstract terms about infinities. But set theory provides a means for precise terms like finite sets, infinite sets, countable sets, uncountable sets and the whole variety of different infinite cardinalities.