It's cool. It happens to us all.
1. Yes. Even though I've never had a major reason to use it when I have the water seems clearer. I wanted UV for green water and/or killing anything messing with the fish, but I haven't had a single problem since putting the Sunsun on that tank.
2. Zero flow reduction for me. The way it is integrated into the canister design doesn't affect the flow rate at all. The Sunsun I have with the UV on it has 4 media trays so it's really tall and powerful. I have it on a 35 gallon hex and haven't cleaned it in at least 7 months. The flow has slowed, but still breaks the surface of the tank so no point in cleaning it yet.
3. It can't hurt to buy the filter w/UV, but not required to be happy with your Sunsun purchase.