Will Baude, as usual, is full of interesting questions. (Sticklers, I hope, will put aside the fact that while one can be full of many things, questions are not such things ;) .) The question in question:
(Brackets his.) Generally, yes, I think accidental allusions are still allusions, but it will depend on the nature of the accident. If, in a poem about rusty nails, the first letters of each line coincidentally spell "William Blake," there's no allusion. But if the author is addressing the stuff of past literature (subjects addressed in past literature), and the author has read such literature, then any instance of channelling literary themes in such a way that can reasonably be considered unique to a certain author is an allusion to that author, intentional or not. This is so because the heart of an allusion is not what it tells us about the author's intentions, but what new thematic channels it allows to flow into the work, what other literature it brings to the fore as a compliment and counterpoint to the alluding work. And, I assume that writers are so steeped in their cultural and literary heritage that allusions are bound to spring from authors who gave no explicit thought to that heritage.
Of course, whether an allusion is accidental still matters. Use of the phrase "a modest proposal" may indicate that a work is meant to be satirical, but not if we know that the author intended no such reference to Swift.