Ahem.
There is no Colossus in the BP continuity. The Colossus project died when the Colossus got beamed to death. Now, you have observed that the GTVA is holding back. They aren't. They are sending every last ship they can spare to Sol, so that by the start of WiH, there are three Battlegroups operating in the system. They can't send more ships without weakening their standing defense commitments elsewhere, and given that the stated reason for going to war is rather thin at best, they cannot allow their populace to start doubting now. If they were to weaken other fleets, they would undermine their main argument for going to war in the first place.
Second, there is no blockade. At the beginning of WiH, the GTVA has firm control over the node and the outer system.
Third, the Solaris class is incredibly powerful. While they may take a while to whittle down the Collossus' HP until it asplodinates, the Colly will be disarmed and disabled long before then. Same goes for the Karunas and Narayanas, what they lack in terms of sheer DPS output, they make up in precision. The only reason why the UEF isn't steamrolling over the GTVA fleets is because of politics; up until the battle of Artemis station (AKA WiH's opening cinematic), only 2 of the three UEF fleets were actively conducting offensive combat operations.
As for Durgas and Vajradaharas not making a difference, you're pretty much very wrong there. A single wing of Vajras can utterly annihilate any but the biggest capital ships fielded by the GTVA.
I'm not so sure the Colossus would be useless in the Sol Theater, it would simply fill the UEF public with terror and dread. Here they are struggling hard again Raynor's and Titan's, then suddenly this gigantic, gritty, ugly tev juggernaut appears through the node. Bristling with enough firepower to rival nearly half an entire UEF fleet all by itself? Terrifying to your average 'buntu. Granted, in direct combat a Colly would struggle with it's Capella-era green beams. But the psychological impact of the GTVA possessing such a vessel and using it would be very beneficial to undermining the UEF's will to fight.
Besides, I'm sure Steele could do very magical things to the UEF if he had command of a Colossus. 
NGTM-1R is very right when he say the Colossus is a blockade breaker. It is neither made nor suitable for stand-up line combat, especially against a foe who is just as maneuverable, and has the ability to track the Colossus in real-time. "Feint, Parry, Riposte" only worked because Koth wasn't aware that the Colossus was operational and in his system. The UEF, on the other hand, is far better at gathering intelligence, and thus at evading the Colossus until it can be suckered into a trap. The name of the game is subspace chess, and one of its rules is that the most powerful piece is one that has not yet entered the field.