My impression is that the Renjian wasn't on station for reasons that might well seem shockingly naive in retrospect: the Elders were well aware that some form of reunion was imminent, but they were all convinced it would be peaceful. It's an open question whether they'd actually tracked the node down at that point (the lack of science ships or a waiting delegation suggests otherwise - nodes in BP are not always very predictable in their motions, especially after 50 years of disconnection), but when the Renjian did arrive on the scene, it did so in the following context:
For the UEF’s part, the arrival of the 14th was not unexpected. The Elders had greeted the sighting of GTVA probes with open arms, and in spite of a Fedayeen (the paramilitary black ops unit reporting directly to the Elders, often - and arguably correctly - labeled 'state sponsored terrorists' by the GTVA) report suggesting that the probes were stuffed with sophisticated ELINT gear, prepared no military contingencies for the GTVA’s arrival. The widespread assumption was that Earth’s lost brethren had achieved a degree of peace and enlightenment similar to that which pervaded Sol (barring certain elements of the Kuiper periphery and the military). The coexistence of Terran and Vasudan technological elements in the probes scanned by the Fedayeen was cited as evidence for this view.
Only when the newcomers failed to arrive in a timely manner did the Fleet Admirals (Calder, Byrne, and Netreba) convince the Council of Elders to prepare a limited military response. Admiral Calder also initiated a ‘training exercise’ for the Third Fleet, loading several frigate divisions with live ammunition and practicing quick-response jumps to various points in the system. When the 14th finally did arrive, the Renjian responded to reports of a massive subspace transit with conflicting orders. The Elders’ standing request was that the visitors be escorted directly to Earth. Admiral Calder privately requested that Captain Leicester hold the newcomers at the node as a measure of caution.
When it became apparent that the newcomers had hostile intentions, Captain Leicester reacted rashly. Believing the GTVA warships comparably armed to his own Karuna, Leicester engaged the Orestes while calling for the 3rd Fleet to respond. He promptly found his ship gutted by the Orestes’ plasma beams – the first occurrence of a tactical nightmare that would plague the UEF for the rest of the war.
But as for the question of what else the Renjian could have done, Captain Leicester reacted rashly. He believed he could survive the engagement until the rest of Third Fleet arrived to seize the node. He was very, very wrong, and he didn't survive long enough for relief to arrive. But if Third Fleet had in fact deployed in force while the 14th was still in chaos, they might have taken the node. The four frigates Calder deployed were basically routed in a panic, and the attackers' failure to press their advantage only contributed to the confusion, since it gave still-hopeful elements of the Council of Elders a reason to argue that the whole thing was some sort of colossal misunderstanding.
From a clear vantage point with only a few elements to think of it's easy for us to have better ideas. But for Captain Leicester, scrambled to the location of the new node to respond to reports of a massive subspace transit, 'don't give up the node' and 'keep them tied down here, instead of tied down bombing Earth' were probably first on his mind.
The real pressing question in my mind is this: given that beam damage is applied per-frame, and given that Karunas cause awful lag, how could four Karunas fail to win the day?
