Right, and this is where so many regulations go wrong, the end punter not interested in products/materials from sustainable sources - how on earth do you want us - respectable retailers demanding from their suppliers products that conform with this regulation - to ever compete with those...
The catch: the quality of the bonding between top layer and backing, the position of the T&G, plus since beginning this month: will it comply with the EU Timber Regulation?
You get what you pay for
What ever you select, make sure the underlayment you use DOES NOT contain a DPM - which will cause condensation and effect the parquet floor (could lift up in the end)
Forfilling you need clean sand-dust (preferably from the sanding sanding - 80 grit)
After filling you sand again to remove the excess filler from the floor.
HardWaxOils are not suitable for maintenance, they are only use on bare wood. Your floor needs a polish (wax-based), read here for more information about maintaining your floor
Remove as much as you can, if needed throw some in a freezer to make the bitumen even more brittle and easier to chisel off.
Bare wood is the ideal, but a stain of bitumen can be expected. Modern adhesives will take a bit longer to bond them back
You started off with the wrong product on your floor. On tropical wood species you should not apply a first coat of HWO, but a thin oil first - followed by a coat of HWO.
Time and polish will settle things down.
It has nothing to do with applying too much HWO, it has to do with the Teak being "oily" on its own (hence the correct suggestion - too late however - of the rep to use a thin oil for the first coat instead of HWO).
Cleaner liquid is something quite different than the polish I recommend. The...
don't apply any HardWaxOil over it again. The oil in the HWO hasn't had time enough to penetrate the wood (especially the teak) to do its work before the wax in the HWO started to do its work.
Best would be to apply a maintenance polish which will reduce the patchy look after a while.
You can float a wood-engineered floor over parquet as long as you use the proper underlayment, one without DPM.
On the other hand, on concrete you'll use an underlayment with DPM.
Elastilon is a good product and comes without a DPM, so lay a DPM sheet over the concrete first
Most wood blocks with T&G come in left hand and right hand pieces, have you considered this?
If it is a failure of manufacturing, why not consider 10mm blocks without T&G?