My wee little structural engineer agrees that floor is definitely U shaped and nothing we have been able to do is fixing it. Having prepped and painted all the pieces as we should and dried them under weights to make sure they were flat before assembling it has been decided an elephant must have stepped on it.
Well :/ this will be left to dry, I can make a replacement floor later. All the pieces were flat and doing find until they were stood up and put together. If it warps to the point of not being assembled I'll decide between smashing it and smashing it and putting the plans away for later use.
Having not paid full price for this kit what ever the outcome it was an experience. Hopefully the next update will include more walls.