YYA, I don’t think this is the answer. You are looking at it from the benefit of hindsight. As we discussed many other cases – it is not always known what the right course of action is. Mendelsohn, R Hirsh, Besht, Chaim Volozhin, R Salanter, R Kook, Saul Liberman, R kotler – my list is on purpose in no order – they all tried to address urgent issues with some approach.
As one American gadol said: “You miss 100% of the shots you don’t take”. So what unites all of them:
1) they actually tried to solve problems
2) they were criticized for that
I am not a baki on Lieberman, but it seems that he, and many in his time, were under assumption that Conservative movement will be dominant among those who keep some idea of adherence to “halakha” – way larger in numbers than Orthodox in America., so it is vital to work trying to bring that movement closer. This was not a crazy idea. I heard from R Nosson Sherman that his father was urging his elderly congregants to continue coming to the shiur through rain and snow – “because who knows what will be after you”. He did not know that his son is going to publish so many biographies of gedolim that the problem now is not lack of information, but our inability to figure out what is true among so much published ….
And as we see from this apparently recently uncovered letter, Lieberman consulted some unnamed rabonim about this decision.