Shouldn’t have a considerable impact due to dust/splashes of water when the flap is open. There is a mechanism to drain water in case of occasional splashes. However, if there are heavy rains or you’re washing your scooter, it’s recommended to close the flap.
It’s not the fan, but the belt noise. When it rains, there’s likely dust and grime that stick on to the belt. When the vehicle is running, this will cause some sound, but it’ll slowly fade away as the dust and water get ejected out of the belt.
The vehicle is not completely waterproof. The floorboard recommendation exists since the motor and some more sensitive electronics are placed just above that area, and water can affect those. Wading through water when the level is just below the floor board is ok.
It won’t stop working the moment warranty expires. Batteries degrade over time slowly, so you’ll notice that your range numbers would get affected, but nothing significant.
You can see this graph that illustrates how the oldest battery packs being used on 450s on road are performing. The data looks promising, and you can expect to not replace your battery pack for degradation for around 5 years or so. That’s when the impact on range would be significant enough to warrant a replacement.