Alençon could easily be removed from the table by Anne de Beaujeu's husband/comte de Montpensier living longer. Originally, due to tensions between Anne's husband and the comte de Montpensier, who refused to be acknowledged as duc de Bourbon in right of his wife (the match was between Suzanne de Bourbon and the Comte (after his and her father's deaths, she later wed his brother, the Constable), Anne and Pierre were considering marrying Suzanne to Alençon.
But the fact of the matter is, AFAIK, Henry rebuffed Marguerite since he would've preferred the candidature of one of Louis XII's daughters (only Claude was alive then). However, Louis wouldn't bite. The fact that he regarded the dowry as paltry is no surprise, since she would only be marrying as a princesse du sang not the sister/daughter of a king. As to the paltriness of said dowry, compared to Catherine it probably would've seemed small. But, to my mind, the only way Marguerite might end up married into England is if Arthur survives (but perhaps still predeceases his father with no issue).