I've just been skimming this thread and thought I'd add my 2 cents on the OP.
Technically it is possibly to win the war through conventional means. We know reaper ships can be destroyed by conventional organic warships as we see several times throughout the games and their ground forces aren't all that impressive. The only real factor is numbers. Organics just don't have them. If the Asari, the richest, most populous, and technologically advanced race this cycle had built 1 or 2 dreadnoughts a years and say 5 to 10 cruisers then how powerful would their fleet have been after their 2,000 years as a space fairing race? The fact is that the organic fleets are insanely tiny when compared to the populations and territory they are expected to protect. The US alone has more then a dozen aircraft carriers, with ships a 1/3 the size or a dreadnought and crews over 4,000 and that is only with a population of 300 million. How many humans does the Alliance protect,11.4 billion on Earth alone and how many more beyond? And yet they only have 9 dreadnoughts. Their are trillions of Asari and Salarians, they've been in space for thousands of years and colonized hundreds if not thousands of worlds, they should have hundreds if not thousands of dreadnoughts each, instead they have a few dozen. What's strange is that it doesn't take long to build one, in the 2 years between me1 and me2 the Turians build 2 and the Alliance 1 with 1 more finished the next year and another under construction.
But after the Reapers arrive, I don't see the organics as being able to mass produce the number of ships they would need for conventional victory, the reapers seem to target the most powerful systems first so reducing capital ship building capacity. If the network is shutdown then operating on a cluster level I can see some cluster build up massive fleets while they wait their turn for the harvest 2-300 years down the line and using that time to build thousands of dreadnoughts and cruisers to protect themselves. Even then, lore wise their is over 10,000 reapers dreadnought, even if we don't see evidence in game, so while I think they would inflict losses I don't see victory in that scenario. The only way I could see traditional conventional victory after the start of the game considering the default fleets would be if they figured out how to mass produce the thanix cannon and mount it on small cheap fighters giving them the power of a cruiser. I could see major industrial worlds and their supporting planets, cut off from the rest of the galaxy but still having ease of movement within the cluster, building and stockpiling tens of thousands if not hundreds of thousands of these thanix armed fighters 2-300 years from the present and giving the reapers a nasty surprise when they show up. Repeated over several worlds and clusters it might be enough to turn the tide, but it might not. The only other option would be a true anti ship energy weapon that allowed them to completely bypass the reaper barriers.
But that would take far to long for a game to play out.