THE PLIGHT OF A SINGLE DAD... No sooner had they settled in Washington than history repeated itself: the welcoming committee of available women. The casseroles. A widower Alex McKenzie I might be, but he was not looking for a new wife. ' His four-year-old son, however, had picked a new mommy...just in time for the holidays. Next-door neighbor Shannon O'Rourke was striking, successful and single. Not once, though, had she come bearing casseroles. (Her twoalarm-fire attempt at baking Christmas cookies with his boy fquite possibly the reason.) Homemaker material or not, Shannon had resurrected Hiis withdrawn son...when even he had failed. For that, he was tempted to kiss her. I Where was mistletoe when you needed it?