1. yes use the coolant overflow reservoir. Nice to have if you overheat.
2. If your bikeoverheated and spit out coolant maybe your radiators are not full. Pull radiator cap off when it is cold. Start bike and run. Add coolant as it is running to fill it completely. Put on radiator cap after it appears you have filled all you can fill.
3. Check jetting, float valve, idle speed, etc to make sure it is running properly. If you lock up the rear brakes without pulling inthe clutch you can manage to stall it, but it shouldnt be easy to do.
4. As torquey as the WR450F is, I dont understand why the clutch would be killing you. You should not have to clutch it a lot unless you are on really tight single track. If so, you might have to lower the gearing to reduce the clutching required. What front and rear sprockets are you running? Or did you mean the clutch pull is really hard and requires too much effort? If so, check the clutch cable, lube, adjust as necessary. If that doesnt do the trick, you might have to pull off the clutch cover and see if you have other issues going on. It really should not be difficult and not require going to Magura hydraulic clutch.
If all this is a result of really difficult trails, then you might want to consider a Rekluse autoclutch.