Okay, so thought I would post an update as to how this is going.
Adding the batting back to the other side was not near as bad. For starters, I taped the backing down. I didn't do that for the first side because I was only dealing with a small portion of the quilt - and I have done similar size layers without taping the batting down and it has worked. But then again, I normally start with the batting laid out, smooth the top over it, then peel back the top to spray the batting for basting. Slightly different procedure. And I am usually on the carpet, which helps hold things. Different on a wooden floor.
Then I laid out my batting, easing it in, being very careful not to stretch it. I started from the center out to the ends with the iron on tape. Then I pulled back the batting and sprayed only a small section to begin with, down the length of the quilt, and carefully smoothed the batting over it while kneeling on the completed side of the quilt (so I was behind the batting smoothing it forward over the taped down back). I did 2 more sections this way, then did the same for basting the top down to the batting.
And while I won't say that it was fun, it wasn't the torture I felt it was the other night. I didn't get frustrated, and it went much faster. But I caused most of my own grief because of the way I chose to do things the first time around. After all of that, I would consider splitting the backing again in the future on a larger quilt - since I have a better idea about what to do and not do.