The meaning of bread and butter is unknown (apparently)
I don't think it's a rowing term - or I'm sure Martin Gardner would have said so, and it seems from The Annotated Alice that it's not known what (if anything) the phrase is supposed to mean
Gardener suggests several possibilities, but has no more knowledge on Carroll's intended meaning than you or I do...
Re the sheep being a white piece - I think, in Wonderland/Looking Glass terms, being 'a bit cranky' IS relatively kind
EDIT Lenny has some interesting stuff in the Resources section on the White Queen. In Alice on the Stage, Carroll explains a little about how he perceived a selection of the Wonderland characters. This is what he says about the queens:
(White Queen bolded for clarity)
Let me cull from the two books a Royal Trio--the Queen of Hearts, the Red Queen, and the White Queen. It was certainly hard on my Muse, to expect her to sing of three Queens, within such brief compass, and yet to give to each her own individuality. Each, of course, had to preserve, through all her eccentricities, a certain queenly dignity. That was essential. And for distinguishing traits, I pictured to myself the Queen of Hearts as a sort of embodiment of ungovernable passion--a blind and aimless Fury. The Red Queen must be cold and calm; she must be formal and strict, yet not unkindly; pedantic to the tenth degree, the concentrated essence of all governesses! Lastly, the White Queen seemed, to my dreaming fancy, gentle, stupid, fat and pale; helpless as an infant; and with a slow, maundering, bewildered air about her just suggesting imbecility, but never quite passing into it; that would be, I think, fatal to any comic effect she might otherwise produce. There is a character strangely like her in Wilkie Collins's novel `No Name': by two different converging paths we have somehow reached the same ideal, and Mrs Wragg and the White Queen might have been twin-sisters.
http://www.alice-in-wonderland.net/books/onstage.html