Low-Light Focus: Is it me, my speedlite, or my camera?

Hi everyone,

This is my first post ever, so hopefully I do this right.

I'm a college student and I've recently been hired to photograph several events for student organizations. Last night I photographed a sorority formal, which was in a very dark banquet hall. This was my first time doing something like this.

My equipment includes a Canon Rebel EOS SL1, Sigma 18-35mm f/1.8, and a Neewer NW670 speedlite. I was in aperture priority between 2.5 and 2.8 most of the night. Single shot AF (to use the AF assist beam from the speedlite).

All in all, I got a lot of nice shots, but there were some shots, which after loading them into Lightroom,  was shocked to seem the focus was off. For instance, the glass on the table was tack sharp but faces were blurred.

I'm saying all of this because I want to improve my "in-focus percentage" in low light.

My Canon SL1 only has one cross-type focus point, compared to something like an 80D with 45 cross-type points. I like to do focus and recompose using backbutton focus, but I couldn't seem to do that with the speedlite; It always wanted to reacquire focus when pressing the shutter. I don't know how the Neewer speedlites compare to "real" Canon speedlites.

With a speedlite attached, I basically hold down the shutter and pray that it's going to focus correctly. I hate that feeling. I wish I could either have more control or more of a guarantee that the shots will be in focus.

Any suggestions?

Thanks so much in advance. (Sorry for being so lengthy)

Tim