Black Limestone laid before sealing

[Quinnci]

We're currently having a wood burning stove installed and have chosen black limestone slabs for the hearth.The builders doing the fireplace have laid the slabs before sealing them. I have been told that the stones will be sealed on the day the stove is installed.
After doing some research, I realise that firstly the stone should be sealed before laying and that 2 coats need to be applied before the stove should be be installed. So, I've bought some LTP colour intensify and sealer.
What are the consequences of not sealing before laying? they have dyed the cement black but have left the edges at back open (not cemented to the back of the fireplace) with a gap of about 5mm. should we be grouting this.
any thoughts would be welcome as we need any problems sorted before Monday when they plan on fitting the stove.
Thanks

[PHG Wetrooms]

The reason the tiles should be sealed before laying is just to reduce any cement staining that might occur.
Obviously it's too late to worry about this now, is there any cement staining on the slabs
Can't understand why they would leave the gap at the back as this will just be a dirt trap - would definitely fill this and then seal asap (assuming it is clean and stain free)

[Quinnci]

Great, there's no cement marks so was going to clean with hot water and then seal. Is it ok for them to fill in the gaps after sealing?

Thanks for help