Hello Avtar,
Let me start by stressing that it is Einstein's "conclusion that gravity is not a force" (not mine) and this conclusion is an established fact in modern physics.
I will answer your questions, but the best way to understand general relativity (GR) and gravitation is to answer those questions yourself by taking seriously the four-dimensional (spacetime) view of the world. Only in the framework of this view one can have genuine understanding of gravitational phenomena; GR showed that only spacetime physical quantities adequately represent the world. Einstein's initial reaction to Minkowski's spacetime physics was negative, but only after Einstein adopted spacetime he was able to arrive at GR.
So your first question is answered - it was Einstein who introduced both the principle of equivalence and the explanation that gravity is not a force, which demonstrates that there is no contradiction between the two (see also the second paragraph in the quote below).
I am sorry that I will have to post again part of what was already posted on this page, but the answers to your 2nd and 3rd question are given there:
"The experimental fact that falling particles do not resist their fall confirmed the geodesic hypothesis in general relativity, which in turn explained why the particles do not resist their fall - the particles' acceleration in the curved spacetime surrounding the Earth is zero, which means that no force is acting on them and they move by inertia. The observed apparent acceleration of falling particles is relative, which is caused by the fact that the particles worldlines and the Earth's worldline converge toward one another (this is called geodesic deviation). Therefore the observed apparent (relative) acceleration of falling particles is caused by the non-Euclidean geometry of spacetime in the Earth's vicinity and is not caused by a force.
When a falling particle hits the ground it is prevented from moving by inertia and it resists its curved-spacetime acceleration (that is why the force of weight is inertial in general relativity in full agreement that there is no gravitational force). The measure of that resistance is the particle's (passive gravitational) mass. So general relativity also nicely explains why inertial and passive gravitational masses are equal."
I think the following analogy, often given to illustrate the essence of GR (that no gravitational force in involved in gravitational phenomena), is most helpful. Imagine that a distance separates two observers whose locations are exactly on the equator. They decide to move north by following a path that is perpendicular to the equator. As they move and monitor the distance between them they discover that they are approaching each other. If they are unaware that the Earth surface is spherical they would think that they move on a flat surface and the only explanation of their getting closer would be that they are subject to some force. The truth is that no force is acting on them; simply they move on the surface of a sphere.
So the fact that the observers approach each other can be explained by two hypotheses - (i) either they live in a flat world and there is a force between them, or (ii) no force is causing their mutual attraction since they live on a curved surface (this analogy is best understood in terms of spacetime when you consider the worldlines of the observers which converge towards each other exactly like the meridians converge towards the North Pole). A crucial piece of evidence can help the observers determine which hypothesis is the correct one - the acceleration between the observers while they are approaching each other is exactly the same no matter what the masses of the observers are (by contrast, a given real force accelerates different masses differently). Falling bodies of different masses fall with the same acceleration.
I hope this helps. Best wishes.