Your post is bang on, Blackchubby, as it exposes the most dangerous issues right now.
I am not sure how to describe Biden's posture, which looks comatose, but so far must be described as caution. If the US were to get involved, what is the strategic aim, the end result? Given that most of the damage in Ukraine is being caused by artillery bombardments from inside Russia, could NATO get involved without bombing targets inside Russia?
The irony here is that Russia's land forces have been so so poorly organized and equipped, that bombing Russian targets may lead to a Russian defeat more clearly defined than the kind of war fought in Iraq or Afghanistan, which begs the question, does the US want Russia defeated in a war? It might remove Putin from the stage, but it doesn't mean an equally aggressive Russian nationalist will not replace him. These 'known unknowns' are exercising minds in the Pentagon, hence the restraint.
As you know, a Casus Belli has to be more precise for NATO to act on the basis of its charter, and so far, Russia has not provoked that kind of reaction. The military action has the political objective of establishing without question Russia's sovereignty over the region that comprises Ukraine in the West but which, because of historic Russian Imperial history could also include the Baltic States and parts of Poland. This on the basis that Belarus and Moldova are compliant to the extent that 'they know their place'.
The assumption is that if Russia were to face defeat or another year of fighting in the Donbas it would seek an escalation of its 'special operations' in the Baltic states, my view being Putin might try a single attack on one of them just to see if NATO does in fact act, or expose the perception it has that the US doesn't want to get involved in any more wars, so Russia can do what it likes.
For this reason, your reference to the food supply issues brings in the maritime aspect of this conflict which has not received much exposure, perhaps because it is more likely to provoke a direct confrontation with the Russians than threats to the Baltics.
Lawrence Freedman lays out the scenarios in the link below, noting that Russia's attempts -successful so far- to control the maritime links between the Sea of Azov via the Kerch Straits to the Black Sea and through the Dardanelles to the Mediterranean, are illegal but that an international maritime force designed to protect Odesa and facilitate its exports, would be the maritime equivalent of a 'no fly zone' over the air space of Ukraine that the US has explicitly said it will not enforce. Turkey's role in this too is hard to read, as Turkey tends to act in its own interests regardless of its NATO membership (cf Cyprus) and would have to agree to any maritime force protecting shipping in and out of Odesa.
Will the 'war at sea', if it happens, be the inevitable 'war with Russia'?
One rogue factor could be Boris Johnson, desperate to remain Prime Minister in the UK and perceiving Germany and Macron to be NATO's 'weak link', sending a Royal Navy vessel into the Sea of Azov- but let's just let that thought sink somewhere, given the state of our Navy as it is, let alone Johnson's declining powers.
For this reason, I think there is/will be pressure from the EU and NATO on Zelensky to at least negotiate terms for a Ceasefire, if Ukraine cannot stomach a treaty of any sort that cedes territory to Russia's control. Biden wants to avoid the US becoming involved in a military conflict anywhere, Putin knows this and is deliberately provoking the US, and I think Biden's posture remains committed to not getting involved, but while this appears to benefit Putin, it only does by forcing Ukraine to some kind of settlement that Putin can call a 'triumph' even if it means his original strategic vision has failed, along with the decimation of the officer class and substantial troop -and maritime losses.
So yes, either there will be a military confrontation at sea, or there will be a messy, unhappy compromise that forces Ukraine to concede to Russia, though whether that solves the question of grain exports I don't know, as Russia is stealing grain and selling it on the world market, and though Ukraine can, with difficulty export grain via rail and road through Poland, the disruption to the production and export of grain and other things will continue, pushing up prices, but an issue which Putin doesn't care about.
"Protecting commercial shipping is by no means a simple option. Escorts would need to include minesweepers. Accompanying warships can also suffer from mines. There would need to be unanimity in Nato to authorise the operation – Turkey in particular would need to sign up. Because of the Montreux treaty, it has an effective veto as it would need to authorise Nato warships moving through the Turkish straits from the Mediterranean to the Black Sea. And Turkey’s actions are not always predictable."
Russia's Black Sea blockade causes food shortages for the whole world - New Statesman
On Ukraine's exports-
Odessa official: Ukraine needs help to break Russian blockade | World Grain (world-grain.com)
Russia has blocked 20 million tons of grain from being exported from Ukraine : NPR