Looking at the scenario:
-Nightmare Monsters are pourting out of liminal cracks everywhere. Renxiang's attacks are supereffective against Dreamstuff.
-We have higher realms raising a wall and manning it. Wang defenses are static, Luo forms a harrying force.
-Polars likely highly disturbed, their defensive measures are readied fortunately, but they will require considerable reassurance.
-Ling Qi is likely too barbequed to do much in the way of diplomacy for the last segment.
-The supply of nightmares is not quantity based but duration based since the source is in the Liminal, and it will last as long as the Sovereign fight does.
--This isn't the worst of it yet, thats when Deming dies and all the power bound up in him is unleashed.
Option 1: Renxiang moves out to blast as many as she can.
-Opponents are dispersed, there is no large concentration of targets for her to get a decisive opening strike, she would have to deliberately place herself as bait to get such a thing.
-This does not take good advantage of Wang defenses, as it moves the focus of the attacks outside those defenses.
-This enables Luo harrying to work more aggressively on clearing the field.
-There is a high chance of Renxiang needing to tap into Liming for this, and the resulting incapacitation, rendering her unable to do any diplomatic smoothing.
-Lone Hero rushing out to do battle with impossible odds is highly regarded by Imperials, but is more associated with Perkunas for the Polars. Useful to scoop up extra achievements domestically, but unlikely to help convince the Polars that we're sane, reasonable people when the sanest, most reasonable persons they've interacted with just did a YOLO.
-Stamina issue may be a bigger factor, Renxiang is going all out immediately, but the monsters will keep spawning until the Liminal battle is over.
Option 2: Renxiang fights as part of the defense formation
-Impact is limited, Renxiang's role here would be to break up concentrations of enemies where they threaten to overrun the defense, or the harrying forces.
-This maximizes the benefit of Wang defenses, as all key targets are within them and they'd have to be overcome first.
-This keeps Luo forces on a conservative posture.
-Renxiang conserves Liming usage, and is likely going to be able to participate in the final part of negotiations and deal with the inevitable questions from the Polars.
-Renxiang would be performing As Expected for both Imperials and Polars. Useful to act as a stabilizing force in the aftermath, less useful to scoop up lots of personal credit.
-Renxiang more likely to be in condition to still act if there is a final nasty surge.
[X] …And Cai Renxiang remained on the earth, letting loose her light with caution, in close coordination with the harried White Plumes left in the lower valley away from the conflict raging on the cloven mountain. Relying on their defenses to preserve what was important, until the tide receded.
As such there really is no reason to take option 1 unless Renxiang is hard up on prestige.
This is tower defense, flying out in front to nuke the first wave isn't productive, however cathartic it may be, we don't need to go gloryhounding.