It is fairly clear that for Scotus, haecceity is not any relation between an individual and something else. This seems clear from his reply to the 'negation' theory of Henry of Ghent discussed in Question 3 (nn 49-56). 'Nothing is absolutely incompatible with any being through a privation in that being, but rather through something positive in it" (n 49). He gives the example (n50) of there being nothing present to sight. This does not produce any incompatibility with the sense of sight. By analogy, if being indivisible were simply a negation like not having anything present to sight, there would be no contradiction or incompatibility in something that is extrinsically indivisible being intrinsically divisible. So individuality is not a form of negation, and by inference not any form of relation. Individuality must be intrinsically a feature of the individual. "It is necessary through something positive intrinsic to this stone … that it be incompatible with the stone for it to be divided into subjective parts. That positive feature will be what will be said to be by itself the cause of individuation. For by 'individuation' I understand that indivisibility - that is, incompatibility with divisibility". (n57)
That leaves the other difficulty raised by Paasch. If haecceity is an absolute, positive, intrinsic feature of this stone, why is not contradictory for God to create another individual with the same haecceity. What specifically about this feature makes it, in Scotus's words 'incompatible with division'?
The answer to this probably lies in the sections of distinction III (nn 48, 76, 165) where he explicitly says what he means by haecceity.
In section 48 he says that we must not ask what it is by which such a division is formally incompatible to an individual (since it is formally incompatible by incompatibility), but rather what it is by which, "as by a proximate and intrinsic foundation", the incompatibility is in it. What is it in this stone by which, as by a proximate and intrinsic foundation, it is absolutely incompatible with it to be divided into (subjective) parts? (See my note on 'subjective parts' here).
In section 76 he says that individuation or numerical unity is not the indeterminate unity by which a species (e.g. man) is said to be one species. A designated unity is a 'this', that which it is inconsistent to divide into subjective parts. The cause is asked not of 'singularity in general' but of this designated singularity, i.e. as it is determinately this.
In section 165 he says that which is a 'this' is such that it is contradictory for it to be divided into several subjective parts, and contradictory for it to be 'not this'. It cannot be divided by anything added to it, for if it is incompatible for it to be divided of itself, it is incompatible with it, of itself, to received anything by which it becomes 'not this'. To say that something can be this and that through something extrinsic that is added is to say contradictory things.In summary: haecceity for Scotus is an absolute, intrinsic feature of an individual thing. It is the feature whereby we conceive of and signify a thing as this. For any this, anything that does not possess this feature is a not-this, and thus not the same individual. Thus it would be contradictory for God to create another individual from this one with the same haecceity as this one. To have the same haecceity it would have to be this. To be a different individual had would have to be not-this. Contradiction.