Unfortunately, there is a ton of nuance to this. There is a difference between the number of threads used to generate a frame (for filters and compositing), versus the number of threads used to encode a frame (by libx264). If encoding with H.264/HEVC, the encoder will saturate any remaining CPU and make it look like it’s being used well. But Shotcut itself may only be using a single core to generate the frame.
The easiest way to verify this is to use an export codec that is near-zero CPU load, like hardware encoding or libx264 with preset=veryfast and then we can see the cores and threads used to generate a frame.
The other nuance is which filters are used. Some thread very well. Some like “Reduce Noise: Wavelet” only use a single thread (but it’s awesome at its job so I use it anyway). Using a zero-load encoder is the main way to tell what Shotcut is really using, as opposed to what the encoder is using.
Since you noticed your CPU usage at 70% instead of 100%, that tells me the encoder is waiting for a frame to be generated and is sitting idle.
This used to be true. GPU was disabled many versions ago due to instability. It can be manually turned on, but few people do it. Not recommended. Those were probably old discussion topics you found.
Whatever makes you happy lol. 
For real-time editing, this would be true. If you edit with proxies, the proxies are low disk usage and there is no speed issue with affordable giant magnetic drives. I still use large magnetic HDDs since I use proxies. Proxies are amazing.