"The order of most primitive to least primitive (mammals) goes like this...

Monotreme
Marsupial
Placental mammal"

This is wrong - you are assuming placentals developed from marsuipals and marsuipials developed from monotremes - this is not currently supported by the fossil evidence.

Evolution is not a ladder or a cone-shaped bush with the simpler and older forms at the bottom and more advanced forms at the top, with one species being more advanced than another; it's a branching, sprawling climbing plant throwing out runners in many directions with many dead ends pruned by natural selection. In the mammalian runner placentals, monotremes and marsupials are all offshots of different twigs, not buds on a single twig with monotremes at the bottom and placentals at the top. They are the result of diversity within a monophylectic group, not a indication of evolutionary status as 'primitive' or 'advanced'.

The old ladder view, taught for so long in schools and still in some textbooks now has long been shown to be defunct; the idea that if a genus is older it must be simpler is not taken seriously by modern students of palaeontology.

It might be worth having a look at the wikipedia page on cladistics which explains it far better than I can (although it does use the term 'tree of life').

Of course, when the aliens finally land they can prove all this wrong with their far superior knowledge (assuming they have DNA etc).