Is it safe to use denture toothpaste on real teeth?

No. Denture toothpastes are more abrasive that those meant for natural teeth.
Test. No. They are usually more gritty. Take some regular toothpaste and rub it between your fingers, now do that with the denture paste and notice the difference. Please use only regular tp on your teeth.
Not recommended. Denture toothpaste may not have Fluoride in it. While it probably wouldn't be unsafe it would probably be unwise. Stick to regular toothpaste for teeth.
Sure. Won't hurt them. But , likely no fluoride, so won't help as much either. It is ok to use regular toothpaste on dentures. Forget the nonsense on tv. It is best to soak in efferdent-like solution which kills bacteria, but perfectly fine to brush them with regular toothpaste.