For motor nerves to be affected without injury is hard to believe there is nothing to worry about.
but if you have back issues, you DO have trauma of lower motor units. Just not denegnrative but physical one due to compression/inflammation.
you do not have the most important part of ALS - neither prominent clinical issues, nor UMN signs in addition lolower motor neuron damages. And all your read is filtered throug huge filter of fear in a way to say you that you are sick.
I did not hear what exactly your first neuro told you after quite a pressure you put on him, and, what is most important, let me tell you that most probably you also may not hear him exactly. This is a bit like news headlines are made: it says "Arnold Shwarzenegger solds hot dogs for living" - you read it in a deep amusement just to found that Mr. Terminator plays a hobo in his new movie. Each word itself is clear and correct, but they make you to perceive an imaginary situation as a real one. So most probably your first doctor also did not mind to tell you what you feel he told.
Doctors do not have any reason, neither medical, not personal or ethical, to hide such important information from you, mostly becasue they might be accused for that lately. If they say you are Ok, then you are OK. If two of them, of which one is an MND specialist, say you are OK - then you are really physically OK and only your spirit needs attention (which is a huge job to do believe me)