Negotiate at the most effective level. Between the US, Russia, and Ukraine. It would be wise to include input from China.
Who should do the negotiations? Pres. Putin and advisors of his choice Pres. of Ukraine and advisors of his choice US: not Biden or Harris or Blinken. Maybe someone can suggest a suitable statesman to take that role, who is not overly biased or idological, but fair and pragmatic?
Dream on!!! Look what it took to stop Germany, Italy, and Japan in WW2. One single man starts a war and one single man stops a war. Negotiators are part of the fog of war.
When Russia is attacking the evacuation corridors during the hours it negotiated that they wouldn't be attacked, one can only assume one of two things: Either they cannot be trusted to keep their word or the diplomats and the military are not under control of the same person.
It takes two reasonable people, or two reasonable sides, to negotiate. Right now, one side, Russia, has not been reasonable at the negotiating table whereas Ukraine has.
Open the option to use military force. That's the most humanitarian way to end this. Biden's strategy is bleeding the Ukrainians out slowly. They are doing better than expected so far, but their military is broken up into pockets that can't reinforce each other and Russia now controls the energy supplies. If this drags out Russia will win militarily. If the US says its willing to send troops into Mariupol Putin would back down. He can't back down now because it's embarrasing to be at a stalemate with Ukraine. But it's not as embarrassing to be at a stalemate with the US or better yet a 30 nation alliance. He can settle for Donbas.