Surfing on a Floating Floor - Learn me engineered wood over a really wavy subfloor

I'm helping a friend of mine refurbish her pole studio and now that the demo is finished, it's time to start figuring out how to put it back together.  The previous engineered floating floor was put down about 15 years ago over the carpet that was already in the room and it started showing some pretty serious wear about a year or two ago.  There was a pretty significant amount of flex to the floor and in some areas, the heels of the shoes were able to crack and puncture the planks.  Yesterday, I pulled out the wood and the carpet and got it down to the plywood.  This is a pretty old building that is not really in the best of shape and apparently shortly after she moved in there was some sort of dehumidifying operation on the floor below her which she said caused the floor to buckle pretty badly and it's never been the same since.

With the subfloor exposed, you can really see how wavy it really is.  It looks like you're gazing across the dunes if you put your face at floor-level and look across the room.  As I took up the old planks it was very obvious that those planks had air gaps between them and the subfloor pretty much everywhere.  The new flooring is actually a commercial product and is about 12mm thick as opposed to the residential-grade 8mm thick planks that used to be there and it does not have the underlayment attached to the planks so I'll have to roll something out, which is fine. 

I considered some of the leveling compound, but there are a few areas where the floor drops down about an inch-inch and a half and we're probably talking about a literal half-a-ton or more of leveling to do the whole 800ftft room.  And it's on the second floor, so I'd have to carry that thousand pounds up the steeper-than-modern-code staircase.

If I dropped 3/4 tongue-and-groove OSB over top of that, would that flatten it "enough"?  The other problem with the floor is that not only is it un-flat, it's also un-level, so would I risk having the edges of the OSB panels coming together at angles that would create problems?  Can I glue down the underlayment and then glue the plank to that?  (I assume that wouldn't work on a floating floor)  Just level out the worst spots with the mortar or some shims?  The good news is that my instructions aren't to achieve perfection, but to get better than the cracked up floor and a 5 year lifespan.  So knowing that "perfect" isn't going to happen and that the project requirements are to have a floor that will last for at least 5 years, any advice on how to mitigate the worst of it?

If you have an inch of sag between joists it's probably time to talk about ripping out the plywood and starting over with new decking. If that's not going to happen, I would be filling those dropped areas with layers of plywood or osb to build them roughly back up, then putting down a new layer of decking or underlayment over the whole floor. 

If the plywood is arched up between the joists I think I would either be pulling it all or cutting out the worst of the raised stuff and decking over it.  Arched up is going to always flex and squeak though, and that would bug me.

I installed 550 sq ft of engineered floating wood in my last house but it was on a pretty flat concrete slab.

All of the manufacturer's requirements I've read are fanatical about moisture and flatness.

Maybe failure is an option and other types of flooring should be considered rather than fighting an uphill battle. 

In reply to wae (Forum Supporter) :

Do you want character or a flat floor?  If you put 3/4 tongue and groove on a wavy floor it will turn into a wavy squeaky floor as the fresh wood pulls the nails loose.  
I want a floor with character. So I deliberately allowed some joists to vary and used rough sawn wood joists. In rooms I wanted quiet floors I drilled recessed holes and put screws in covered with wood plugs.  I was careful to match the color and shade of the wood plus made sure the grain was in the same direction. It more than doubles the install time.
In rooms I wanted to hear the squeaks I just nailed it in the normal way.  If it was too noisy I'll sneak a few screws in with careful plug selection and alignment. 
I want character but not shouting. 

In reply to frenchyd :

I've made well over 100 trips to Minnesota over the years and most of them have have been to the Minnetonka area which I believe is a quick drive to your incredible home.

Once we get back to normalcy, I'd love to treat you and your lady to lunch, dinner, whatever, and get a chance to check out the amazing home you've created over the years.

Sorry if I'm being too forward but the more I know, the more I want to experience it first hand.

I think the gypsum self leveling stuff is your best bet if the structure is solid. 

In reply to RX Reven' :

You're welcome.  Please stop over.  But not until the risk is past. At our age/condition we need to take every precaution.