First off it's not an early warning radar. You're actually turning on your targeting radar. When you power the system up and put it into Running Mode, you're activating the Tall King radar. The three position covered switch is for the targeting radar.

2. I set it for mach 3 as that's roughly how fast the SR-71 is flying.
3. You can't really detect the SR-71 with your targeting radar until it's ~180-220 km and acquiring it ~180 gives you plenty of time for the Nonius, while also reducing the amount of time you're on air. Not to mention you can't engage the SR-71 until it's ~150 km anyways.
4. I'm almost positive the S-200 uses a cooling system for it's missiles. Anyways, if you spin up the missiles when the SR-71 is ~200 km you'll probably be launching them within 2 minutes.
5. The reason I only use 1 missile for this engagement is simple; no US aircraft has ever been fired on by an S-200 before. Therefore, the SR-71 will never even know it's been engaged and thus won't be maneuvering/evading. Of course in real life I'm sure the CO would've ordered 3 missiles fired.