Changing to a Mixer Tap on the kitchen sink

yes a deck mounted sink mixer will be able to be fitted to most sinks with two taps.

Yes absolutley , just opt for a deck mounted mixer tap, which will have the same 'legs' as your individual pillar taps, the holes are manufactured at a set distance to allow this , the same as bath tap holes.

Put any tap you like there if it's a single hole you can by a stainless steel cap to match the sink, just make sure the rubber seals are fitted correctly so it won't leak into the cupboard below