I need help understanding flattening transparency options

What happens when you leave the balance slider on the vector to 100 if you have a drop shadow?

I have a very large file for a backdrop that I've added a drop shadow to two of the elements. In the past, even though they request a illustrator file, it would seem that not all the design team knows what to do with drop shadows. Shadows and since I've never used them in illustrator, assume they're going to be a problem.

I can't find exactly where they put the drop shadow. When I open the object I see a white object box and when I click on it it only removes the blur. I don't see where the actual drop shadow is.

Anyway, I'm assuming flattening transparency would work, but I don't understand some of the options despite looking on the group.

When you move the slider balance? What preserves the vector file and what is needed to keep the drop shadow as it is shown? 2 years ago I had issues with a file that had special shadings on some 3D effect in illustrator. Eventually I had to give up on illustrator and make it in Photoshop and just imported as a graphic. The rips software that the printer was using did not translate it correctly.

I'm assuming they're going to expect me to do something with the drop shadow, but I'm having issues right now trying to export the 6 GB file to a PDF. Maybe that's not possible and that's why illustrator iss hanging?

I have been able to make the AI file with the PDF but I don't know where that lives and I open up another topic on that. But when I tried to export as a PDF to see what it would look like, that's where I'm getting the hanging.

So if I'm going to upload the. AI file for them to look at and tell me what they think, I thought maybe I should flatten that transparency where I put the shadows. Everything else is either raster or vector.  So I'm assuming that will all remain as I see it.

But I don't want to reduce the quality and just don't know what leaving it at vector balance at 100 does compared to 75.

All I know is this morning just one small drop shadow on a set of books that were vector and trying to move them around becomes a problem because it keeps trying to redraw the shadow and even just moving two books in the image caused the system to lock up several times. I'm surprised I got this far on it. To be honest. This is one of those things that once they print it, it's something that I don't want to have to fight them on. Redoing. Their proofs are terrible. So I want to give myself the best shot possible by simplifying as much as I can. I wasn't sure though I wanted to rasterize everything since they do work with illustrator files.

I'm not an illustrator artist. I buy clipart and composite things for my show. So please understand I don't know all the lingo or ins and outs.