DensPlus (and other non-paper-faced gypsum panels) are much more resistant to moisture than the green paper panels. That's why they can be used as tile backers. However, Dens Plus is much harder to prepare for painting than green board; it needs a skim coat. Frankly, other than in a bathroom, I've never used anything other than standard drywall in basements. But I always have a dehumidifier running to keep rh below 50%. And keeping the drywall a few inches above the floor and using XPS under the baseplates protects the wall from minor flood damage. Sealing the interior of a block wall--to the extent that the sealers can even do that--just means moisture accumulates in the wall and possibly causes damage.

Sorry, but without the use of an appropriate underlay, carpet is a poor choice for a basement. There are just too many sources of moisture. Even if bulk water is under control, the cold concrete is the perfect surface for condensation when warm damp air comes in contact with it. As well, there is always a moisture drive upwards from the high pressure under the floor to the lower pressure in the air above. Appropriate carpet underlayments include: Dri-Core or competitive products; Delta FL or its competitors followed by OSB or plywood; and extruded polystyrene followed by OSB or plywood, as shown by Building Science Corp.

You MUST HAVE a vapour barrier installed over the concrete. All floating floors have this requirement. Vinyl is NOT to be used as protection from moisture migration UP! You can purchase 6mil vapour barrier + moisture resistant tape (aka Red Duct Tape) for the seams. Should cost about $0.12/sf for the space. And NO....you CANNOT use another "underpad" under that vinyl. You can ONLY USE polypropylene plastic sheeting. Vinyl does NOT like padding underneath. You must work with something thin WITHOUT padding or cushion. Go cheap and cheerful. The 6mil vapour barrier (as above) is considered "Best Practices" in the flooring industry. No need to reinvent the wheel.

If you are concerned about the possibility of mold/mildew, then the most effective thing to do is fix the concrete so that it doesn't have a problem. You could spend $2-$4/sf on materials (such as DriCore) only to have a damp slab underneath. Which means the potential for mold is STILL THERE. Or you can fix the problem at its source: seal the entire slab properly. That means shot blasting, etc. Which isn't much more money per square foot than the $4/sf for the EXPENSIVE DriCore. And if you read DriCore's website, they will require a 6mil vapour barrier UNDERNEATH their panels if the hydrostatic pressure is higher than 'x' lbs/1000sf. That's for the 'regular' DriCore. To get away from this requirement you have to use the HIGH PRICED foam panels. And then you have to purchase OSB/Plywood to go over top of the foam panels....etc. Industry Standard means just that. The industry (flooring) recognises that slabs almost always contain moisture. The control of that moisture is up to the homeowner...but the flooring professional can take steps to ensure the product THEY are installing will have the best chance of being successful in that space. That means stopping moisture from 'hitting' or reaching the floor above. What happens underneath the vapour barrier is up to the homeowner to deal with...because the homeowner is ultimately responsible for his/her cement slab. The professional has done his/her job by using the industry's regularly used product such as 6mil poly sheeting. Again, if mold/mildew are your concern (and it should be) then the 'fix' is not going to be some panels sitting 1/2" higher than the concrete. The fix is going to be the professional sealing of the slab. Shot blasting is a b!tch! The epoxy sealants are a b!tch to work with. The primer is noxious and the microtopping is a nightmare. Which is why it is best to leave the sealing of slabs to the professionals.