I can't really tell from the pictures but if that screed is just a layer over floorboards also you can probably just remove it by breaking it up then overboarding, otherwise you are just relying on a bit of screed over some boards for the strength on that spot, i'd maybe want to know what's under there and why they screeded over it. I don't understand why they previously would have created a high spot with screed as that would have caused a lump for the laminate flooring also.
As for grinding you would need a grinder with a diamond cup. I have only really used weber leveller as it whats i get locally so i can't recommend which is best for you, i think you can get mapei leveller from screwfix.
Yes that would be good practice, it will only aid you in getting a more level floor for boarding over, also securing any loose boards with screws also.
Without really knowing what is under that patch of screed i'm a but reluctant now to advise just tile from that to meet the backerboards, there could be hacked up boards or damaged boards under there and potentially a weak spot waiting to happen.