...all Civilization will be in mourning, yes. As soon as dath ilan is sufficiently sure that all of these events are not themselves carefully crafted attempts at mind control, to release any information about the Summit to people who haven't been trained to resist mind control. At some point after that, people in Civilization will probably start to be happy again, but this will probably take more like at least a week than half an hour, especially with the massive reallocation of economic effort to projects like "building arcologies to house millions of abandoned children" providing a constant reminder to everyone of what changed and why it had to change.
There's no reply to the question about why dath ilan doesn't want the Bobbis thinking they're friends.
(To the general room:)
It's looking to dath ilan like they should probably withdraw from this summit. People here, according to recently received information, are behaving in ways and doing things to one another that dath ilan considers deontologically forbidden to engage in, especially in a First Contact situation; but also, dath ilan shouldn't shut that down when engaged-in by other planets, if the potential end result is saving the lives of six-year-old children faster and with fewer net deaths than any more direct intervention permitted by portal logistics. It looks to dath ilan like dath ilan's presence within the summit proper may be interfering in the progress of that possibility.
Dath ilan continues to be ready to contribute infrastructure, wealth, planning, etcetera to any efforts led by Firstplanet or Green; though dath ilan does have any verification conditions before they send over anything that was really expensive to Civilization.
There will continue to be a dath ilani embassy that receives guests, but, for now, nobody traveling back to Civilization (including any dath ilani who've left it).
Dath ilan wishes they could say more, but other planets don't follow the basic mathematical rules dath ilan thought would obviously govern how to communicate about these things; and in fact, some planets here don't seem to have affordances for communicating with them at all, without people getting very upset, unless dath ilan tries to behave in ways that dath ilan itself considers forbidden.
Dath ilan is interested in scaling trade with Firstplanet, Green, and Ozytopia. Ozytopia can expect an infrastructure-building mission shortly. Elfland and Malachitin are still in the running as potential trade partners and may (if they wish) consider themselves to have open, friendly lines of communication with dath ilan.
Here are some culturable samples of a dath ilani bacteriophage which is very safe, minimal side effects, and which bacteria have a very hard time evolving immunity to. No intellectual property payment is requested; the group that invented this wanted it to be a gift to every planet encountered, no matter what else was going on with them, even if it looked like they were about to invade dath ilan, and Civilization voted to lock that decision into place no matter what happened at the summit itself. Dath ilan doesn't know if anyone else will understand the intention behind that - it doesn't make mathematical sense, obviously, and is in fact insane - but it means something to dath ilan.
Everyone gets a copy, obviously including the Bobbiverse.
Dath ilan is sorry.