To answer your first question...You can try your luck with the seedling if it's at a decent price. With Siamese mango being from SE Asia, it is most likely polyembrionic, meaning that all seedling sprouts except for one (zygote) are identical (clones) of the mother tree that the fruit (seed) was grown on.
Those are decent odds that you will get a clone (1/2 to 6/7).
I have found polyembrionic seeds to yield anywhere between 2 and 7 sprouts.
However, there is a post from Simon about variations that can naturally occur with these clones.
Hence, it still is better to buy a grafted tree, which will produce fruit faster than any seedling.
I have not seen any other nurseries that sell this Siamese mango variety. Not even sure if Zills HPP even propagates this variety. If anyone can get this variety for you (and at a decent price), it would probably be Squam256.