The US must bet on deterring Russian threats against Ukraine, or decide to back down to them.

I think Putin is bluffing and will not attack if the US takes purely defensive actions. He is an opportunist and grabs whatever is easy to grab. Furthermore, all his threats are only hints; if he does not attack, he will not have lost face. Thus, I think the US should take the defensive actions, then negotiate.

A commitment not to invite Ukraine into NATO would not deny Ukraine sovereignty, so I think it is ok to include in a peace deal, provided this is matched with Russian concessions. The US could demand Russia respect Ukraine's sovereignty by withdrawing support from the Donbas separatists.

However, demanding return of the Crimea would be too much. It is too late to undo that act of aggression.