1) You can't convert to an FAA Commercial Certificate. You can use 61.75 to get a Private Pilot Certificate, which will work as the base for a Commercial.
2) You will need an instrument rating - this requires a Knowledge Test and a Practical Test. You can work the required 61.56 Flight Review into this training.
3) You can do Single Engine Commercial or Multiengine in either order. SE first is cheaper. Do the SE Commercial training from the right seat. Then the "repeat the checkride" for CFI. ME add-on is pretty straightforward. I did Glider to ME to SE. This past weekend I did the checkrides for SE Commercial and Instructor back to back. Commercial and Instructor each require Knowledge Tests - Commercial Pilot, Fundamentals of Instruction, and Airplane Instructor.
4) It might sound a tiny bit crazy, but if you also do the instrument work from the right seat, then you can work with your instructor to train for CFII as well. It's too expensive to not combine where possible.
Good luck!