... might think was grape shot due to the small balls looked more like grape than the proportionally larger balls of true grape used in the navy...
I think canister would be more effective against the ranks of infantry used during the period than grape.
I think the comments you made about canister (case) and grape being easily confused in contemporary reports is the clue. I think I read this same comment somewhere else. Therefore, when reading an actual account of a battle on land or sea, I guess caution is the word in accepting the expertise of the writer.
Both are anti-personnel. In my mind, the larger shot (grape) would be more effective slightly further away (like a decently close ship-to-ship battle) and the smaller shot (canister) would be more effective very, very close. This "might" apply to both land warfare and sea battles.
I suspect that, as you noted, canister was more widely used when a land canon was in danger of being over-run and grape was used to spray the deck of an enemy ship, crowded with boarders. But maybe, both would be effective, to some degree.
Wikipedia has some information that kind of dances around your question without being definitive but that might be a place to start checking for sources.
In "The arming and Fitting of English Ships of War 1600-1815
" by Brian Lavery, both grape and canister (case) are discussed as being used by the RN. Lavery hints that canister (case) may have been used earlier than grape by the navy but both had their day. This might just add to the confusion.
I'm sure that other books discuss naval ammunition and still other books discuss army ammunition but is there a book that compares & contrasts both? I would be interested in that also.
And then... there is the similar type of anti-personnel shot called "langrage" which seems to be similar to canister (case) but consisting of scrap metal pieces instead of small round balls.
In "Naval Cannon
," a booklet by John Munday, drawings of case, grape, and langridge [sic] are shown.
Note: modern anti-personnel rounds are "flechette" rounds similar to canister (case) but consisting of hundreds of small "nails" with the heads formed into metal "feathers" creating a small metal dart or arrow. I've seen such rounds fired in demonstration. Ouch!