Originally a civet was thickened with the blood of the animal at the end of the cooking. Using a little flour instead does not spoil this rich and wellbalanced stew.
If you buy venison from a good butcher, it will not be old and tough and will not need long marinating. In fact, if it is farmed venison, it does not really need marinating at all. I find you get a better flavour if you marinate for 3–4 hours rather than overnight.