All sounds good. I think your recommendations seem good. the 4" panels are a good idea and 16 of them would be a good number if they also add curtains.
In my experiences with PAC's (I'm with you now) there was always a good amount of absorption in the form of curtains. Many had big cavities where the organ pipes once were or still are. Is this a more modern building with a lot of reflective surfaces instead?
What was your sound source for the measurements? In small rooms there is no real issue with using a directional speaker, it doesn't really matter much. In large spaces like this, the use of an Omni sound source becomes important. Since I'm more of an amateur acoustic consultant rather than a true licensed professional, I make due with what I can. I've done one of two things. If they have extra speakers, I've created an Omni array. This is nice as its a more full range source. I do this with just 6 speakers mounted so that they point up, down, forward back, left right. You probably get the idea. Since most places don't have such things laying around, what I do is use a large balloon taped to a stick and pop it. The balloon needs to be where the sound sources would normally be, so I would try a few stage locations and just above the orchestra pit. You can buy the balloons at a party store, bigger ones make a more full range impulse, so I would look for big ones and fill them up nice and big. That helps ensure the sound is loud and full. I don't use REW when I do this so I'm not sure if REW can handle this kind of external impulse. I would think so. I know it can handle an external sound source.
If you do the Balloons, you may want to then use their sound system as well, since that tells you specifically what their sound system is doing in the room, but the balloon test isolates the room issues and also is good if you have non-amplified sound sources.
I personally have very limited experience in spaces this large, I've typically worked with other experts rather than being in charge, so much of my suggestions are what I have seen recommended and make sense to me based on what experience I do have. It's possible a true professional would have something different to say.
There are a number of calculations that are common in performance spaces that REW doesn't support. You can do most of these using REW and some external calculations however. Some require different software, so I use a mixture of Audiotools on my phone and ARTA. Audiotools, ARTA, and REW collectively cover everything I need, and each allow files to be exported for analysis. Examples of measurements that may require something like ARTA and some external anlysis are IACC measures. These require two mics.
http://www.bnoack.com/index.html?http&&&www.bnoack.com/acoustic/concerthall.html
This is a good reference if you don't already have it. It may have some useful information.