If "completing the set" was so important, Renxiang herself would have reached out to the GF Ducals and requested a representative to be sent over.
If the Guo had wanted to have a presence in the Summit then we wouldn't had had a reason to refuse, nor the capability to do so. But going out of our way to invite them to the first negotiations is just awkward
On that line I don't think Ling Qi has the authority to request anything of them, even through Han Jian.
Really, this is something Renxiang herself should have done a month or two ago. Just a letter requesting and envoy from the Guo. Though I guess this is something inevitable in quests, people no doing things expected of them so the users can decide and have agency
I'm not really worried about the "leaves Golden Fields isolated from the project" bit. It would be awkwad to decide something so important in the long term to a sudden vote during an informal meeting.
The trade of glass and skystone for sunblood metal is too beneficial to ignore, and that's not to mention possible colaboration to tackle and restore the Grave.
So they may no be present during the Summit itself, but there is no problem with them starting contact once it's clear that the diplomatic relations are stable.
[X] Wanted to him to raise the possibility with his own connections of whether experts on Twilight corruption could be enticed to speak with the Seared lands experts, as a bargaining chip for negotiations. (Easier ask of Han Jian, keeps Summit as is, leaves Golden Fields isolated from the project.)