Everything you have said is wrong, either contradicted explicitly by the QM of the quest, or by the mechanics of the quest.
The Infrastructure Cost has of the Galileo no bearing on its "cost" in any real sense, the number we produce, or anything else. The only scenario in which it is relevant is if we were still building the Galileo and attempting to start producing the another heavily armed capital ship at the same time... which we aren't going to do for various reasons. Essentially, the Infrastructure Cost represents the production lines we have available for weapons; not cost in an accounting sense, and is only relevant in terms of tracking whether we're trying to build too much at once. The post you quoted that line from went into this, summarising all the conclusions from asking
@Sayle about it, so perhaps take a closer read.
The agility of the ship (or lack thereof) is taken into account in its Tactical Rating. The Tactical Rating is the ultimate descriptor of how good the ship is in a fight relative to its weight class, etc... Given the the Galileo with torpedoes has a Tactical Rating of "A", then it is
by definition a highly capable combatant. See also the Single Target Damage Rating, which also factors in the ship's manoeuvrability and is a whopping 50% higher. Even the Multi-Target Damage Rating (also inclusive of agility) is a bit higher. Being less agile is certainly an issue when using torpedoes against smaller more agile ships, but less so against big ponderous capital ships, which there are no shortage of in this era, like the Klingon D7.