1. the gravitational force acting on a falling body and its weight is constant. but the law of universal gravitation tells us that the gravitational force on a body increases as it gets closer to earth’s center. is there a contradiction here?

It's not so much a "contradiction" as an approximation. Newton's law of gravitation is an inverse square law whose range is large. It keeps people on the ground, and it keeps satellites in orbit and that's some thousands of km. The force on someone on the ground - their weight - is probably a lot larger than the centripetal force keeping a satellite in orbit (though I've not actually done a calculation to totally verify this). The distance a falling body - a coin, say - travels is very small, and over such a small distance gravity is assumed/approximated to be constant.

We can say that the force required to hold the monolith is same as the component of weight along the inclined plane

here we have

m =76 ton = 68946 kg

now force required to hold this