No, it is not a valid theory, as I pointed out, it still does not answer the question of how life started, it avoids it, as if somehow life could start spontaneously on some other planet but not here, or even worse that it could start on some frickin' comet rushing around in a vacuum, without water, oxygen or other life giving nutrients. It is a BS argument and if you can't see that you are blinding yourself to reality.
Really. Comets are full of water and organic materials. Spores can freeze and remain viable for a long time. It is hypothetically a quite valid theory. You just don't want to hear it because it demonstrates you can separate the evolutionary history of life on Earth from how life started on Earth. And, like I said, you can still have a debate whether there is a process that created life or whether there was a higher power. IMO your goal is to try and discredit the basic concept of evolution with this approach - if science cannot explain how life formed, therefore the geological record of increasing complexity in life forms is bogus. That is just not the case.
Comets are mostly water. Oxygen is toxic to some of the ancient bacteria in Archaebacteria, so it is not a vital nutrient. There are ecosystems on Earth underground or around volcanic vents that do not require sunlight and photosynthesis to start the food chain - bacteria synthesize chemicals such as sulfur compounds and that is the foundation of the food chain.