Well I just looked on ebay and the cheapest 970 strix was at $145, "reserve not met", and did not include shipping. Not sure you're going to find that card for $150 total, yet. Other than the Witcher, how are other games performing? Seems like a 970 should handle 1080p quite well, and I certainly wouldn't buy more hardware for one game. If you plan to stay with the 970 for a few years, yes an SLI solution would make a huge difference. Ignore all the naysayers who will chime in about the horrors of SLI- I've had SLI 670s for three years and 95% of the time it works perfectly. Your PSU is more than up to the task.
"Is running the fan at 99% degrading my video card?"
No, the fans don't mind full speed at all. Cards degrade faster from heat, so run those fans all you like.
"Should I overclock my CPU even though my build doesn't have additional cooling?"
Some will scream no way, not without an aftermarket cooler and yes that is recommended. But it's not like your CPU is going to shoot up from 45 degrees Celsius to 105 in an instant. Just back down the OC, or return to stock speed altogether until you have better cooling if it runs too hot. It is IMPERATIVE that you download a CPU temp monitor that shows in your taskbar at all times. I use this freebie: https://www.techpowerup.com/realtemp/
By the way, OCing your CPU is only going to get you a 5-10% increase increase in fps, max.
"Is buying a used 970 STRIX (on ebay, craigslist, etc) worth it at this point?"
Sure, SLI 970 would give you close (but not quite) 980ti performance, plenty for 1080p.