First of all it should be understood that an object in orbit represents a higher storage of energy in velocity and altitude. At the very least that energy has to be transferred to the object.
But that's not enough, of course, we will end up using much more energy that will be lost in being transferred to the reaction mass that is needed to obtain a reaction force, and also in overcoming air resistance and lost in inefficiency during energy conversions.
Chemical rocket fuel serves two purposes. It provides the energy from a chemical reaction. But it is also provides the reaction mass.
The more energy you have, the less reaction mass you need. And the opposite, more reaction mass needs less energy.
The energy that is transferred to the vehicle is proportional to the reaction force times the duration the force is active. The reaction force obtained from an engine is proportional to the mass of the reaction mass times the velocity by which it is ejected. But the energy that is needed to eject the reaction mass is proportional to the mass times the square of the velocity.
So if you can do with twice the reaction mass you only need one fourth of the energy, if everything else were the same. But that's not the case. More reaction mass means more mass to accelerate, so this is a differential equation.
The scramjet space plane provides two things here. It mainly uses air as reaction mass, it can use lots of reaction mass, and it doesn't need to bring that itself. It only needs to carry the fuel for energy. The idea then is that it would pull up, coast and use the built up kinetic energy of its speed to break out of the atmosphere. The idea is theoretically attractive, because we use air as reaction mass to get out of the atmosphere.
But we end up using lots of energy anyway, because that's a big aeroplane with heat shielding, large tanks and big fuel guzzling engines that we propose to fling out into space.
I think already X15, but even more White Knight and Spaceship One, hints that all that is great overkill of expensive and complex technology. Instead carry the rocket fuel needed high enough instead. And we don't need to fling the whole aeroplane into space either. An smaller rocket plane will do. The rocket fuel instead represents the energy of the mach 5 speed.
Static air pressure affects the design needed for a reaction engine to be effective. That's why jet engines have variable exhausts. And that's why old space rockets and even the shuttle switched engines progressively as it reaches higher altitudes.
So once we get into space the problem becomes smaller. No longer does friction constantly deplete the energy that we have already built up, and our rocket engine can also be optimized for operation in vacuum. Now we "only" have to reach safe orbital speed and altitude which I believe is at least 400Km.
Earths gravity does not lessen at all significantly. The phenomenon that is experienced is lack of weight, not lack of gravity. Please note that carefully.
Instead, as we increase speed outside the atmosphere, more and more of the gravity is used to curve the trajectory to keep the vehicle in a trajectory following the earth. We need less and less rocket force to compensate our flight direction to counter the "fall" that gravity drags us to. Thus the g's we feel from the rocket represent less and less a compensation for effect of gravity.
Similarly, the energy spent is less and less wasted on compensating gravity.
In that respect, the effect of gravity lessens.
We cannot "feel" the gravity though. What is felt is the accelerating force from the rocket engine.
When finally in orbit, we are constantly "falling" freely, due to the effect of gravity. But since the earths surface curves away from us, we never loose altitude. We are in orbit.
The apparent lack of gravity in space is due to free fall. On the ground, you feel a force from the ground that is stopping your "fall". That's how you "feel" the effect of gravity.
You feel the force that is stopping you from falling. That is the force that is weight.
Once you stop the rocket engine in a space vehicle, you will no longer feel the force from the rocket engine accelerating you. The space vehicle will then be "falling", even if it still moving upwards. It is decelerated and then later accelerated by gravity. There is no force that stops or interferes with this. Thus you cannot feel any force and thus you experience the phenomenon of weightlessness.
The case of being in orbit is identical, only since you're not loosing altitude due to the earths surface curving away from you, you cannot fall down on earth again. But you are falling freely. That is why there is no weight.
The earths magnetic field is not strong enough to carry any vehicle.
Heat shielding is much, much lighter than powered reentry. It's no contest at all. Also heat shielding, since passive, is much more reliable and safer.
Traveling farther away from earth involves building up enough speed to break away from Earth's gravity, and adjusting the energy state that is represented by our position in orbit around the sun. That's one reason why it's difficult and needs large amount of energy, making chemical rockets almost useless.
Remember a vehicle coasting in the solar system is not traveling in a straight line and constant speed! It is in free fall relative to the Sun! So fuel consumption is indeed a concern.
Another is the distances and travel times involved. Again we want higher speeds. Nuclear propulsion with very high exhaust speeds but small amounts of reaction mass (plasma jet) is the solution. A compromise might be solar power driving the engine. But it will be less effective farther from the sun.
The case of getting into orbit is different. Here we need strong forces to quickly break out of the atmosphere and break away from the "loosing altitude" effect of gravity. Large reaction masses are needed for that, so there's no point in not using chemical fuel.
