I'm not sure that by 1850's Russia remained grain supplier up to the same degree it was in the early XIX: almost immediately after the Napoleonic Wars ended Britain introduced the corn laws (or was it grain laws?) which limited foreign imports and remained in power until 1840's. Which, IMO, indicates that dependency was already down (but eventually there were some domestic troubles over the higher bread prices). Personally, I never used "King Grain" argument which would imply that grain from the US could be a decisive factor. But it could be a noticeable factor.
By pure tonnage in the 1850s Russia was the larger supplier than the US, but this didn't see the British fall over when they didn't import Russian grain. It seems that it would be unlikely for grain to be a major factor in any hypothetical war in the 1860s.
The difference goes well beyond of what you wrote. Number of the Russian ports was quite limited comparing to those of the Atlantic coast and, what's even more important, it would be quite easy to maintain such a blockade (but never really happened) because all the way to the late XIX more than 90% of the Russian naval trade had been conducted by the foreign ships of which the British and French represented a majority. As a result, the allied squadrons could go at will to a specific port, do some bombardment (this mostly applies to the Black Sea and Sea of Azov) and sail away or to take hold of an isolated fortification (like Kinburn or Bomarsund). With the multiple American ports and a big merchant fleet the Brits would have to take blockade much more seriously.
However, the results on the Baltic Sea were anything but impressive. The defended ports on the Baltic Sea had been protected by the combination of land fortifications, mine fields and screw-propelled cannon boats (it took 90 days to have 32 of them built and ready for the operations on Baltic - each having from 1 to 3 68-pounders capable of firing the bombs within a maximum range of 2.6km (and precision shot of 700m). With the main artillery of the forts being 48-pounders (bombing) with the range of 2,13–2,8km which forced the British squadron to fire from the distance of over 3km with practically zero results. Sveaborg was bombarded for a while but not destroyed and attack on Kronstadt never even tried with any degree of a seriousness.
In any hypothetical war with the Union in this period the Admiralty had plans laid to blockade all the major ports and inlets and the 1,200 miles of coastline that supplied the Union war effort. Either by winnowing the number of ports (seizing Portland) or by close blockade against the major ports. About the only one which might be daunting is New York due to the naval facilities there and the two entrances which need to be watched. The Union trade would, virtually over night, cease to exist. A small number of Confederate naval raiders caused a loss of IIRC something like 50% of the Union merchant marine either to direct loss or reflagging to escape raiders. The sudden declaration of open season by the largest navy in the world will be catastrophic.
I haven't read anything about screw gunboats for the Russian fleet (evidently they provided enough reason to stand off from Kronstadt, though that was also a portion of the British and French disagreeing on strategy) but you are correct that the British generally only attacked fortified places when they felt compelled to, and with middling success. However, the destruction of Kinburn ought to be seen as instructive, since the introduction of ironclads fundamentally changed the game, making landward defences (especially purely masonry ones) nowhere near as effective as they had been. True the Americans have their own ironclads, but they can't protect everywhere and there's no guarantee they will be as powerful against British inshore ironclads.
This makes sense but as you said the process would be messy and requiring a good calm weather. The coaling ships would be staying in the open ocean which makes them vulnerable both to the weather (like a half-decent Nor'Easter) and to the sudden attacks by the small ships going across the shallow waters. Not impossible to maintain but far from being ideal.
Not necessarily requiring calm weather. Coaling depots could be established on the islands, and coal ferried back and forth or performed closer to shore. Not as efficient as doing it in a proper port, but definitely doable. Coal isn't going to be the bottleneck in these operations.
I'm afraid that you somewhat misunderstood what I was trying to say. Of course, it was possible to obtain the maps but these maps do not make it possible to led the big ships through the shallow waters and the deep water passages quite often had been protected by the coastal fortifications. For example, during the ACW the forts protecting Boston Harbor had been rearmed with new
Rodman guns, primarily 15-inch and 10-inch caliber (range up to 5,000 yards for 15-inch and 4,800 for 10-inch) and some 100-, 200-, and 300-pounder
Parrott rifles (as I understand the range of up to 3,500 yards). So your big ships with a powerful artillery should be kept well away in the sea (which may allow the fast merchant ships to sneak through) and, while you still can go through the shallow zones with the smaller ships, they'd be fighting on equal (or worse) terms with the smaller ships defending the harbor.
The British
had the maps is what I meant to point out, and the navigational and charting expertise to be able to mark the waters they'd be using. Some ships could be lost by becoming grounded off the coast, but that's a hazard in peace time as well.
As for big guns, well those can be a deterrent, but aren't a deterrent when the attack must be mounted and the big ships go in anyways. They might sink a few ships, but that's the cost of doing business in war. The heaviest guns aren't even available in larger quantities compared to their lighter cousins, so you're far more likely to see 10 and 8-inch Columbiads (pure Rodman guns were not built in large quantities) going up against 68-pound and 110-pound Armstrong guns of the fleet. An interesting competition to be sure.
I don't know who would win in any particular match up, but with the record of the ACW the attackers can give as good as the defenders, even with mines and ironclads in play.
In really depends on
when in 1863 things go down, many of the harbor defences were still not quite up to snuff. Though again, this won't impact the blockade much, the forts are too far inland from the main channels to prevent the British from blocking them in most cases.