Wilde's confidence does not strike me as that shakeable. Differences there
probably were, but it's hard to see why he would not have either ignored
them, or adapted himself to Oxford usage, unless there were someone
telling him that the Oxford way was "better" or even "correct".
Jespersen does have some interesting stuff in this connection. After
noting that the use of "I'll" expressing mere futurity is not at all
rare in recent English books ("recent" = late 19th/early 20th century),
goes on to note that "The Scotch and the Irish, hence also the Scotch-Irish
parts of the U.S., use constantly *will* in this way. (word order [sic]!)
After some examples, we get into full-blown moralizing, from William
Dwight Whitney (1868), writing of "A reprehensible popular inaccuracy -- commencing in this country, I believe, at the South or among the Irish...is threatening to wipe out in the first person of our futures the distinction between the two auxiliaries *shall* and *will*....to disregard obligation
in the laying out of future action, making arbitrary resolve the sole guide,
is a lesson which the community ought not to learn from any section or class,
in language any more than in political and social conduct."
This is followed by a quote from Curme (1913), who, after an attempt
to make American usage fit within the prescriptive frame (which Jespersen
finds implausible), notes "This should not be confounded with the
development within Irish and Scotch dialect, where the valuable distinctive
meanings of *shall* have been lost -- a most unfortunate result indicative
of less accurate thought and feeling."
Somebody like that must have got to Oscar.