I know the kind of construction you have with your existing foyer floor. Just fasten down any loose spots with a screw-type fastener or hardened spiral flooring nail prior to installing your wood and it should be fine. However, expect some seasonal movement of your maple, no matter what you do to prevent that from happening. I've used standard cementious (portland cement based) floor fillers to float out dips and valleys before installing nail down wood and it's been just fine. However, you must wait for the filler to cure before installing any wood product over it. A good filler for such a use would be Ardex SDP (self drying patch) or a similar product. Self leveling cementious fillers may have too high a PSI rating (pounds per square inch) for you to easily nail through the cured product. Mapei is not going to tell you to use their product for a purpose for which it was not designed and tested. You are on your own when using using a manufactured product when you are "inventing" an unapproved use for it. Moisture in a crawlspace below a suspended floor system of wood construction can affect that floor system. Flooring contractors have devised many ways of dealing with crawlspaces and preparing wood flooring to be installed over them. There is no stock answer as each house and the soil it is built on is a unique construction. You need to make the best guess you can after researching all your options. Search "moisture in crawlspaces" and you'll get an idea of what you need to consider. Best of luck.

I have not used cork, so I can't speak about that. I have used many other underlayments including FloorMuffler. In my personal experience Sound Solutions underlayment is the best for what you are asking about. One person can't hardly lift and carry a 6ft wide roll of this. Three things you need to have to achieve a quiet and fairly solid feeling floating floor. The substrate has to flat,flat,flat. Use a premium underlayment. Pick a quality floor with excellent milling and a stout joinery system, and the thicker it is the better imo. I have installed this way before and it is very solid feeling underfoot. Most folks are surprised when they discover it's a floating floor.

If you are on slab, I'm not sure what you would be nailing TO. Adding in a sleeper subfloor would be very expensive and will still require a vapour barrier underneath. *Technically a glue down SOLID hardwood can be done on slab - but it isn't recommended unless your HIGH END flooring installer has done it successfully in the past. A glue down engineered floor is technically "more stable" in the widths they are suggesting, but I would question a comparison to a 2.25" solid hardwood. Especially if the solid NARROW strip hardwoods are rift or quarter sawn. The same adhesive would be used regardless of plank structure (solid vs. engineered). The RIGHT adhesive (aka. EXPENSIVE glue) MUST be used over slab in TX. And then the proper SPREAD RATE must be used (lots and lots of the expensive glue will be required) as well as the PROPER TROWEL profile/depth will be required. These expensive adhesives ($100/gallon is common) will achieve 30-50sf per gallon. These adhesives will be the vapour barrier. The most expensive ones will have "limitless" moisture content as one of their key features. Even with the TOP adhesives, the installation will still be cheaper than a sleeper subfloor + nail down installation. The sleeper subfloor would mean a lower priced flooring professional can be hired to achieve the nail-down installation. The glue down method will require the higher skilled professional with all the knowledge required to install a glue down wood floor PROPERLY. And the next issue is the HVAC system. Regardless of the installation method, your HVAC will have to have the ability to maintain a CONSISTANT level of humidity. Sometimes A/C isn't enough in high-humidity environments. Sometimes a whole-home dehumidifier is required in extreme humid areas. Please review your HVAC system's abilities to TIGHTLY control humidity (somewhere around 40% is PERFECT...and it must be maintained 24/7 for the next 25-60 years). Start there. Spend more money for the upgrade on the HVAC system so you can have the wood floors - regardless of width/composition.

Oh dear. Noise abatement requirements with hard surface flooring over light weight concrete (aka "gypcrete"). And you only have enough room for 6mm worth of acoustic insulation. Hmmmm. You are getting into expensive territory. First check what is ALLOWED by the HOA/Condo board. They will have ratings that will tell you what you have to MEET or exceed with hard coverings. Secondly check to see if the acoustic glue underlayments will even grab gypcrete. Most things HATE gypcrete. I mean HATE. There are many high-quality glues that do NOT grab gypcrete. And the one's that do, can be nasty. But don't worry. Being in California means you can't purchase anything that violates CARB compliance - unless you order it over the internet (ahem...best to stick with brick and mortar point of purchase for stuff like this). And just for fun, not all wood floors can be floated and not all can be glued. And not all glues grab gypcrete...see where we are going here? And then there is the noise abatement that MUST be addressed BEFORE you go any further. Without those numbers, you are flying blind and so are we. In the cork industry, I NEVER got my 6mm cork underlay to pass a SoCal condo boards requirements. My customers ALWAYS had to go 12mm with a wood floor (of any kind). You can get some decent sound reduction with mass loaded vinyl. But it is expensive. It is very thin and very expensive. And vinyl does NOT like sitting underneath wood. But it depends on the mass loaded vinyl product. And QuietWalk Plus is a product that is relatively thin and yet has some of the same acoustic insulation as 12mm cork underlay. It will save you the height at least. It too is not cheap. https://www.mpglobalproducts.com/flooring-underlayment/products/quietwalk-plus/sound-rating/