Russia got their warm water port when they annexed Crimea.
The rest of your post brings to mind Prime Minister Neville Chamberlain's September 30, 1938 announcement that he had achieved "peace in our time". Just substitute Ukraine for the Sudetenland.
I have NO faith in diplomacy in this regard. As I stated, best case, "join NATO", to insure the deal and draw our own red line in the sand.
I actually don't see Ukraine as a sovereign country by summer, or shortly thereafter. As for Crimea, sure, but the road around to get there isn't exactly direct, and is that bridge a rail or a road bridge? I believe it's good for trucks only. So not so good for Putin or Russia.
Plus, back to my OP, if Ukraine could manage to join NATO, at some point they will want to take Crimea and the rest of the country back. Putin knows this and probably wouldn't trust NATO to honor the deal forever.
I would like to be right about the best case and wrong about the outcome I expect.