Not asking too much, are you Chris?

"How are planets formed?"

By accretion.

"Why does Phobos and Deimos have such low densities?"

Why do Phobos and Deimos have such low densities? My guess is because they accreted in the Kuiper Belt.

"What would happen to a bunch of small debris in orbit around Mars over a billion years or so?"

Small debris should accrete with Mars, which is why the MRO images were so interesting.

"How did Phobos and Deimos get in to their particular orbits with the rotations they have now?"

The particulars of the capture are beyond me, aside from the general observation that they were captured.

"What are the odds of capture against formation orbital in situ?"

Inifinitely greater.

"BTW, I believe you are wrong about the analyzes of spectra of the first few centimeters of surface material being an indicator of the whole composition of either Moon. My house is not made up of titanium or zinc even though the top layer of paint is white. "

Good point: the spectra of asteroids are now corrected for dust accumulation, and I don't know if the spectra of Phobo and Deimos have ever been corrected.