History:
All work had been undertaken to the customers satisfaction.
Answers:
This would be a electricians job a kitchen fitter would have to get a sparks in by rights
Without seing what tiles you bought its hard to tell. but the tile sealants are for stone tiles the ones that if you spill red wine on it will suck it in. the only way to change a tile colour is to paint them or change them sorry.
If the walls not holding anything ie joists or walls then its not supporting your right. but make sure the full lenth your taking out as joists change direction for many reasons.
Raise a job it depends on where you are in the country.
By law you are not allowed to remove the case from the boiler. the fault on this is inside the boiler so you need somone who is gas safe sorry.