The earth and the moon aren't going to collide PRATT they have a stable tidal lock (well the moons going away slightly) something you can't have.
Remember if the planets don't meet in a nice controlled way you have a big kinetic energy problem and a molten world.
The exact wording you need is "decaying tidal locked planets" ... google it and try working through the problem you are the maths genius ... do some maths. If all else fails start (
http://en.wikipedia.org/wiki/Orbital_decay) there are only a few ways for orbital decay to come about but I know nothing maybe invent some new ways. Remember the planets have inertia spin and have gyro effect makes for a bit of fun trying to solve it all. Thats why I suggest magnets and string but crap what would I know I am wrong.
Once you get through that one genius, when the two planets meet you have another problem. They are spinning draw two circles on a blank piece of paper and draw a clockwise direction on both now look at the two direction of the two sides that contact they go opposite ways.
Consider what is going to happen ... if you cant work it out genius put two wheels on a pivot start them spinning and touch the two wheels together and watch what happens.
So now you have a new problem the planets have to be spinning opposite which is the only way to solve that problem and now you have to go back and fix your orbital decay.
When you get to that point Sir Genius I will give you the next problem ... but hey I don't know anything.
As I have said before point 1..6 are not excluded by giant impact theory they simply aren't covered. Taking what you have said about Einstein and Poincare then you can't say that makes them different.
Wish I could stay around and help but I don't know anything.
Good luck with it all and now I really do have to go .. actual science to discuss.
