Please provide some information on some abs workout which can be done at home preferably without using equipment, and they are not harmful to spine/back/abdomen and are safe to do without trainer supervision?
You're asking two different questions with two different answers.
- What exercises will increase ab strength without much injury risk to the spine?
- What exercises will reduce belly fat without much injury risk to the spine?
To get the easy one out of the way, there is no such thing as spot fat reduction (outside of surgery). Abs are made in the kitchen, not the gym (for 99.99% of people. Somewhere there's that one person with 5% body fat but abs so small you still can't see them).
If risk is an overriding concern, that implies to me that you've had issues in the past. That would affect what's risky for you more than anything else I can think of. I messed up my shoulder doing shotput in high school more than 30 years ago. I can lift, curl, press,... lots of weight. But every now and then pulling a seatbelt across my lap hurts. Everyone is different.
So, in general, what exercises increase ab strength without injury risk? Well, what do you consider strength and what do you consider risk? Planks are famously un-risky and medium good for building one particular type of strength (static, right in the middle of the muscle range, low intensity, high endurance). Weighted decline situps are still pretty safe (if done more or less correctly) and very good for building a different type of strength. But both are either indicated or contraindicated based on different injury histories.
Whatever you do you'll probably want to also exercise your lower back and obliques, not just the abs you see in the mirror. I personally like Dr. McGill's big 3. They work for me, they might work for you.
Try things out, build weight slowly, see what gives you the strength you want, and doesn't hurt your back. If that's too general, you should ask someone who can actually work with you, who can see your form, and talk with you about what your specific goals and concerns are.
Good luck. And I love your username.