Hi. As you probably have read, there is so much uncertainty surrounding the medical community in terms of what constitutes an accurate ALS diagnosis, bulbar or otherwise. And as you stated, ALS' initial core symptoms can mimic other disorders. I would most definitely consider more than one opinion on the matter, though always expect / prepare for the worst and while hoping and praying for the best. As for the other tests, ALS may not have any noticeable impact on early tests, such as lung capacity or other vital areas.
Typically, a true ALS diagnosis occurs through a process of elimination. There is no specific test or scan that I know of that can outright determine that you have ALS. Only changing physical conditions down that path without another supported diagnosis or explanation.
As a caregiver for a brave lady (my mom) who battled this disease for many years, myself and many others on this forum surely have experienced first-hand that generally there is no clear-cut diagnosis of ALS. From onset, it took my mom almost a year to be properly diagnosed. And even then, we weren't completely certain that the diagnosis was absolute.
I wish you nothing but the best of luck and completely disagree with any diagnosis off of one test from one doctor. Seems like they're jumping the gun to me, right or wrong.