I'm kind of surprised Noam Chomsky is still alive.
"The official position that the war must go on to weaken Russia" is a maybe a bit of a stretch; rather, I think it's more accurate to say the US's position is that the war must be a painful as possible to deter Russia from doing the same thing in Kazakhstan, Georgia and other former Soviet republics. (Or, depending on how crazy Putin and his cronies become, Poland, Lithuania, Latvia and Estonia... but I don't think they're ever going to pick a direct fight with NATO.)
As far as the US position "undermining" negotiations, or the potential for negotiations, goes I think that's also putting the onus on the NATO/Ukrainian side to be "open" to negotiations in the first place. You can't negotiate with the Russian regime; they are demonstrably untrustworthy. The fact that the war started in the first place is proof of this. Chomsky mentions that Macron was negotiating with Putin in the days before the invasion began and at that time Putin dismissed the possibility of a negotiated peace entirely. They had the Minsk Agreements on the status of Donetsk and Luhansk, and those were ignored. They had the Budapest Memo which said that Russia, the US and UK would "respect the independence and sovereignty and the existing borders of Ukraine"; that got broken in the first place in 2014. Why would the Ukrainians trust ANY negotiated peace settlement with the Russians? They demonstrably cannot.
And what do you begin the negotiations with anyway? You let Russia keep Crimea but they have to give up the Donetsk and Luhansk "People's Republics"? Ukrainian pre-2014 territorial integrity has to be absolutely non-negotiable, otherwise what's to stop Russia from similarly cleaving off territory from other neighbours in the name of "saving Russophone populations from 'neo-Nazis'/'Banderites'/etc."? This is what terrifies the Baltics, Finnish and Poles more than anyone: if you let them get away with using this propaganda as justification for annexing territory they'll just keep doing it!