According to the way I understand your conditions, I think the answer is No. In particular, condition 2 seems to suggest that there should be unique maps {1}->{111} and {111}->{1}, and also that those maps be inverses of each other (since there is only one map {1}->{1} and only one {111}->{111}). Hence the map {1}->{111} is an isomorphism, so its truth value is "true" regardless of whether the latter multiset has three or any other number of 1s.
Edited to add: To me a very natural candidate for the category of multisets would be the category of sets equipped with an equivalence relation, whose morphisms are functions on the underlying set that preserve the equivalence relation. In other words, the category whose objects are surjections A->A' and whose morphisms from (A->A') to (B->B') are pairs of maps A->B and A'->B' making the square commute.
The idea is that for a multiset like {1122}, the set A has four elements (like the multiset should) and the set A' only has two elements (like the underlying set {12} does), and the surjection A->A' tells you which elements of A are "the same" and which are different. The commuting square condition tells you that if two elements are equal, so are their images under any map. (So there's no map from {55} to {12} sending one 5 to 1 and the other to 2. However, there are two distinct maps from {5} to {55}.)
This category does have small limits, and the monomorphisms from (A->A') to (B->B') are the ones whose underlying map A->B is injective. However, I don't know whether this category has a subobject classifier, or what it might look like if it exists.