Your problem is that the monotremes actually fit on the evolutionary tree just fine. We have a picture of a line of reptiles called Synapsidia which is gradually turning mammalian before our eyes in the fossil record.
We can see the reptilian jaw double-jointing, coming apart, and becoming mammalian ear bones in this sequence. We can see almost-uniform reptilian teeth increasingly differentiating. We can see the defining Synapsid skull hole forming and moving around, fusing in late synapsids with the eye socket.
What we can't see are soft tissue changes like mammary glands, fur, warm-bloodeness (but we have clues in the bone canals on this one). However, we can predict that these are also creeping in, in some order or other.
That's where the monotremes come in. They're warm-blooded. They have fur. They have mammary glands (but not nipples). But they still retain the reptilian cloaca (monotreme = "one holer"). They lay eggs.
They branched off early. They've been evolving on a unique path for a very long time. They have unique features.
That poison spur you lawyer on is simply a platypus innovation, like its electrosensing muzzle. It's no more impossible than a bee's stinger or rattlesnake's fang, and it's much newer and less efficient than those aforementioned structures. (But a lot of snakes seem to have invented the poison fang independently. You can see some species no farther along in this than the platypus is with its venomous spur.) It's an adaptation of pre-existing structures. (And why is that always true?)
Creationists always try to make the platypus something it isn't.
A designer can do anything. We could find a salamander with hammer-anvil-stirrup ear bones if a designer was fooling around. We could find fish with hair. We could find grasshoppers with feathers.
Evolution predicts exactly the kind of transitionals ("mosaics") that we find. We knew before it happened that we would find dinosaurs looking like birds, birds looking like dinosaurs in the fossil record. (OK, Huxley was influenced by the Archaeopteryx find when he proposed dinos-to-birds in the 1860s.) We knew to expect land-mammals-to-whales. We knew to expect fish-to-amphibians, reptiles-to-mammals, apes-to-humans.
Your Designer is limited to the kind of thing evolution can do and the platypus is no exception.