Copying one of the manlands doesn't make them creatures but I guess you can tap one half of the infinite pool to animate the other half of the infinite pool. Still, a combo requiring three pieces is harder to set up, especially since the lands can slow down your mana curve a bit. It's also blocked completely by Blood Moon. Still a viable strategy though, but I think the normal Deceiver or Pestermite Twin is a better beating outside the combo.
Yup, the plan was to animate enough to generate 1000 land tokens and use 500 to activate the other 500. Nice catch, I had to think about it for a bit to realize how it'd work.
Unfortunately, I don't think that combo is viable in modern; I'm mostly brewing for EDH. As you said, three card combos are difficult, and running two combo pieces that don't do much on their own (Retreat to Coralhelm and Splinter Twin) isn't exactly ideal. I would like to try at least one Retreat to Coralhelm deck in modern, though.
Right now I'm thinking midrange/zoo with Ruin Ghost, Steppe Lynx, Sejiri Steppe, and Knight of the Reliquary. Play early mana dorks and beaters, use Ruin Ghost with Steppe as a bad Mother of Runes, and combo off with Retreat when your opponent taps out. Running a 3-drop that doesn't impact the board in that kind of list seem really awkward, but it'd still be fun to sleeve it up and bring to FNM.