Garage door down, will not open and no access inside
The emergency release cable on my garage door is inside the garage. When it is pulled, the door operates manually by lifting. If the springs are weak, it will take some effort to get the door moving.

You have two questions. 

The first answer may be that your door has weak springs. A lever of some sort is indicated. Consider a two-by-four of some length, perhaps with a steel strap affixed to the end to get under the door. A fulcrum attached to the lever close to the end will give you some lifting to the door. Block the door at the height accomplished, then re-position the fulcrum, perhaps lengthening it to afford additional height. Block the door each time a gain is made. There are also devices known as fence post jacks which will allow straight vertical lifting once you have sufficient elevation to access the underside of the door. Even an old-style auto jack could be pressed into service, but I've not seen one of those in years.

At some point, you should be able to lift the door by hand, as the springs take over the load.

Rather than trashing the door should the above option be out of reach, consider to contact a garage door company for suggestions. With sufficient desperation, many other options are available. My neighbor has a bobcat type loader with fork-lift blades. If my door were stuck badly enough, it would certainly open under the force of the loader.

That leads my train of thought down another track. One could construct a simple set of wedges and tap them into place with a hammer. As the door began to lift, the wedges would be stacked onto additional spacers in order to continue the progress until the springs engaged.

If your release cable snapped and did not disengage the mechanism, it is necessary to perform the lift and remote actions together or risk damaging the opener. When my opener sheared the teeth on the motor, it was strong enough to prevent manual opening but not strong enough to lift the door. Had the release not operated properly, there would have been little to no additional damage to the already worn gear.