Maybe go to the professor on your first day and figure out something that they'll be happy with to deal with the absence. And say it's only 1 class period. Then send a couple emails--one that same week and one the week before the test to confirm.
I have not. I called and got through but was told that I need to wait for a response to my emails. I asked when that would be and they could not answer.
LSAC sets it up and emails you the login details the day you're supposed to sign up for a time slot.
July was a flub, they sent the wrong user names out and had to reemail everyone. It's ultimately the email you use with LSAC.