To do the order 10 one (assuming that an order 10 group exists), note that a subgroup of order 10 contains a 5-cycle and an element $\tau$ of order 2 (which is a product of two transpositions). Conjugate $\tau$ by the powers of the 5-cycle to get five different products of two transpositions.
If any two conjugates contain the same transposition, their product is a 3-cycle, so all the transpositions have to be different.
The five elements of order 2 contain 10 transpositions between them. There are only 10 transpositions available, so one of them must be (4,5). The element which contains (4,5) is clearly in $S$.
For order 12, note that the stabiliser of a point in $A_5$ is a version of $A_4$, which has order 12, and there are five such subgroups, and that accounts for all of the maximal subgroups of order 12. Looking at the elements of $S$, the two 3-cycles fix points 4 and 5, while the elements of order 2 fix 3,2,1 respectively (as given in the question) - which gives a non-trivial element in each stabiliser.
Not sure how enlightening this is ...