The mudras them self are not spontaneous in the true sense of the word. Just like with any for of energy cultivation technique they are fairly specific. Arbitrary chants and hand positions will don't little of nothing for you in terms of energy cultivation. If there are any “spontaneous” mudras it is due to the individuals deeper understanding of how to open the chakras or make the proper connections to promote energy flow. You also have to look at the focus of intent behind these mudras. Simply mimicking the hand symbols and chants will also do nothing for you with out intent.
The point of this only being learned or passed through experience is partial true as well. With qigong, kuji in, yoga or any other "system" of energy cultivation/channeling/focusing anyone can be taught the postures or hand/finger positions with ease. What must be experienced is the results you get from the "proper" application of these postures/positions. These results can't and shouldn't be explained to you, as they are generally what gauge your progression to the next set or the next piece of the puzzle. This is the main reason people MUST have an experienced individual to guide them. Someone who hasn't experienced these things can never teach you the proper way to perform them, as they don't know what results to look for. You need someone with experience to sift through your over active imagination and what is actual progress.
It's late, i've had a long day and the wife just came home ranting about the neighbor kids. Sorry but i lost my train of thought for the moment. I'll try and pick up where i left off tomorrow.
Enjoy life while you can, you never know when things will change.