You rarely need bottom fans in this case, most GPU's block a lot of the airflow anyway. The only problem is when you have a non-reference GPU that doesn't use a blower-fan, they tend to circulate their own hot air because it get's trapped down there. Solutions are using fans to force cool air into the fans and the hot air to the top of the case, but almost the same result can be achieved by making ducts.
Yes. But adding fans make a lot of unnecessary noise. The issue is not being able to control them basdd on GPU temps. Under heavy load it's going to make a lot of noise anyway, but it will make for a more noisy experience when not gaming, which is when the noise is most bothersome anyway. You can get away with not adding any fans or ducts. If you want to lower temps, I would go with ducts. But if you are still in the process of acquiring hardware, go with a blower style cooler. It will serve you better in this case. Non reference open air coolers usually end up a lot quieter in reviews, but they don't work very well inthe M1. Temps seem to be good with the short 970 as well so that is an option if you're not afrer the 980.