Alastor,
I'm so sorry to hear you've received this diagnosis. Remember Laurel's husband who has Lewis-Sumner Syndrome? Initially he too was diagnosed with ALS ( I believe). So, definitely seek out a second opinion. Although, if by your grandfather having had ALS, this means you have the familial type, then that closes the gap a bit (unfortunately). Still, you should have another separate evaluation done.
There is a member here who's son Johnny was diagnosed at age 18. Her name is Valya. Perhaps if you send her a message, she would be willing to give your mom some support. She's not on the forum a whole lot, I have her email address somewhere, but don't feel comfortable giving it out without her permission.
Good luck to you, I wish like anything you had gotten different news. Keep strong hope, research is encouraging, sooner or later there will be something found that will defeat ALS.