Most all adhesives have a memory and the last rows down need to be blue taped or strapped. If you have the extra wood I would advise replacing the gapped rows. None of my clients would be happy if I tried to fill those gaps.

I am certainly not an industry expert but my recollection is that when you lay a wood floor over cement slab you have to use a moisture barrier in the middle. I cannot believe that laying a moisture barrier is more work then all of the work that would go into a tile floor. But I can believe that it might be more costly in terms of materials. You really should research this point - once you decide on the flooring that you want, read its installation instructions- so that you are well-versed and can have a coherent discussion with the GC... I would also ask him point-blank if he has ever done this. It could be he has not and that he does not have a sub who knows how to do it well either...

The condo board (HOA in the USA) dictates your noise requirements. If you are using engineered hardwood you can look at a floating floor installation (to reduce costs...it's one of the reasons engineered hardwood was invented). In most of the US, I've found the requirement for noise reduction is 6mm cork (1/4"). In some of the higher priced condos the requirement is more and therefore 12mm (1/2") is required. This gets into all sorts of issues with floor heights (like through doorways, in the foyer, in the kitchen, etc). Cork is state of the art. Believe it or not, it is pound for pound dollar for dollar the most efficient noise reducer we have. It is the lowest VOC. It is the most economical and ecological of all options in the building world. Yes it is 'old school' but it is also the most viable for 95% of the population. If you want glue down hardwood in your condo, the cork will have to be glued down first. Then you have to wait for the glue to cure (24-72 hours depending on the product) and then the hardwood has to be glued down to the cork. We call this a 'double stick'. The cost of the glue, for each application = $1/sf. Just to glue down the entire thing you are adding $2/sf onto the job. Labour doubles because the glue is two layers (s/he has to glue down a floor twice). A floating engineered hardwood doesn't need any of that. The cork sheets are loose laid (no glue...just edge butted to each other) and the hardwood is floated right over top - same day. Cost of materials goes down (no glue = $2/sf savings) and the cost of labour goes down (only lays the floor once). Labour cost to install floating cork underlay = $0.50/sf. That's it. If you want state of the art, you will have to pay the price for it. You can look at mass-loaded vinyl. But vinyl and wood do NOT play nicely together. You can get some odd odours coming from the glues needed or the two products themselves will have a chemical reaction to each other and cause an odour all by themselves. You can look at the super thick QuietWalk. The acoustic numbers are very impressive but you need to work with the expensive stuff to get it up to condo standards. You can look at rubber but you are going to be spending HUGE amounts of money ($3-$4/sf) just for the low-odour product. Then the specialized glues needed to get rubber to stick to concrete AND wood = another $2-$3/sf. And you STILL risk odour coming from the rubber for decades (every time the sun hits the floor the rubber warms up and begins off-gassing faster). The 6mm cork = $0.80/sf. The 12mm cork = $1.50/sf. That sounds expensive until you compare the TOTAL cost of install (glues, labour, etc). And remember: the adhesives are the single biggest producer of VOCs in the building industry. As soon as you require a 'glue down' hardwood you are introducing glue into your home. To get away from the glue you simply float the floor. With a thick cork underneath the floating hardwood will feel just as solid as a glue down. I'm a cork flooring professional btw. I've seen your situation many times. By trying to go 'State of the Art' many homeowners discover the mess they have made for themselves. It is very sad. Personally I would loose lay the cork (over the properly prepared/flattened slab) and then float the hardwood. It is SUPER easy and achieves everything you want....while saving you close to $5/sf.

Oh dear. That's what I thought. Here's the skinny on 'prep'. Prep is the UGLY stuff that NO ONE sees and NO ONE knows to ask about...nor do they usually OFFER the ugly stuff in their quotes. The ugly stuff is expensive. And in the past they have quoted it and they have lost jobs to lower bidders who did NOT offer prep. Over and over again the 'good' companies offer quotes on prep and time and time again they LOSE the jobs. So they have started to work like everyone else. To stay in business they drop the preparation quotes. Which is how they get jobs....because their bids are now 'in line' with the companies around them. Subfloor prep needs to be paid for. And the company that promises it must deliver it. The basis is, you pay for it = you get it. This company has not quoted nor were they paid for prep work. Sadly (oh so sadly) you were the one who 'got what you paid for' (not meant in a mean way). So...to get preparation done, you will need to pay for it. An upstairs preparation costs = $1-$3/sf for a bit of sanding...a bit of levelling...a bit more sanding. The concrete slab can require $2 - $5/sf for preparation...and in this case the sealant will need to be paid for as well. A slab that is in good shape that is nice and flat, that only needs a good scrub to get a bit of bite to it = $2/sf. A concrete slab that needs a good cleaning (may have old carpet glue all over it) and plenty of grinding and plenty of patching (Self Leveling Compound = SLC) and more grinding can and will cost $4+/sf. And these numbers are why they didn't quote it. The upstairs = $2/sf x 609sf = $1,218. The downstairs concrete (just for fun we'll use $4/sf) = $4/sf x 694sf = $2,776. The prep alone *should have been quoted at $4,000. Their quote should have been more than $25,700. ***At this point, assuming you can prove they were all wrong on all things, they owe you removal of the improperly installed floor. They must replace with 'like for like' subfloor plywood that was there BEFORE they arrived. Be aware that you *might be required to pay the extra $4,000 for the subfloor preparation that you did not pay - that needs to be done...you may not. It depends on how good you are at negotiating. If they won't do this voluntarily you will be required to pay for the NWFA Certified Inspector to come out and document what you spoke about over the phone. And then you present the findings (but do NOT hand over the document...SHE who pays for the report OWNS the report). The findings will push them to do the right thing. Hopefully they will come to their senses before you have to spend several hundred dollars on the Inspection.