The first part is unlikely speculation, given the fact that the Alliance wasn't able to recover from their losses during the battle of the citadel three years prior, it would be unreasonable to assume that they would be able to recover from their loses at Arcturus in mere months. Especially when considering that Arcutus was the only known Alliance shipyard.
And all this doesn't alter my point, even if you disregard war assets as not represenative for the story (which actually is reasonable, considering the many contradictions and inconsitancies) there is still no indiction that the Alliance sufferend any noteworthy casulties.
Well, they have something called incentive and resources. Take a look at what that did for the U.S. Military in the Second World War. In September of 1939, the U.S. Navy was building less than 20 new ships a year. By the same point in 1944, they were putting out more than 30 Destroyers a month. So yes, when put under pressure, the military would be able to churn out more vessels than the average. Would it be the same as WWII? Definitely not. But it would be able to do so.
A tendency that you tend to rely on is the speculative fallacy. You make speculations, defend it as speculation when it's called on, and attack others for speculating and coming to a different conclusion. I very, very, very highly doubt that Arcturus was the only shipyard in the entire alliance.
And going on with your tendency to speculate, there's no evidence supporting that they didn't take casualties. You're using a point that has been acknowledged as invalid to support your claim, and when challenged you stick to your claim without further evidence and hold it to be more correct. Again, I disagree, and state that you have no ground to base your claim at all (and I am not making a claim) and are invalid to say that Cerberus shipright's are poor builders.