Can I felt half a concrete roof

Yes you can as long as you seal adjoining roof I.e get a seal down middle properly or if useing rubber make sure you use a termination bar then youl be fine

Yes you can as I have done quiet a few for the local council when I sub contracted to them

Yes absolutely but you need to use bituminous primer on the concrete first. Then a 2mm underlay followed by a 4mm mineral top layer both applied with blow torch.

It depends on the fall of the roof if yours were the top half running down on to your neighbours you might get away with it. If it’s adjoining then there could be problems with the slab soaking up water and spreading underneath. If he does not want to go half’s it might be better for you to prime his roof when you prime yours before applying the first layer. This will make the water run of and not soak into the slab. Make sure you have his permission first.

No it would be impossible to seal the concrete roof as it will have been cast in-situ and will be one piece! The only way to guarantee no leaks is to fully cover the whole roof.