our plumber correctly installed our wall mounted spouts over the centre of the basin ie water directed to the plug hole but make sure that the basin is mounted forward of the vanity and spout is shorter rather than longer as the water can spurt forward if turned on too hard by unsuspecting guests

The need to seal it attracted my attention because, while I don't know about that particular sealant and things might have improved, the issue of surfaces needing regular sealing is not very sustainably eco friendly and can have health implications. Do you know what solvent/s the sealant contains and what precautions are advised? The other question that comes to mind in relation to your actual question is, how vulnerable might the surface be as it's coming up to the time for resealing?

All personal preference, the only advantage of having them seperate is when it’s plumbed in you can change the distance between the outlet and mixer. With repairs the mixer is a sealed unit similar to an in wall tap-set. You can access the mixer cartridge for servicing by removing the handle and cover plate which wont require tile removal. The only time the mixer leaks from the handle is from a factory defect or if the water pressure is over 500kpa.

I’d have the biggest basin you can have without moving the plumbing. That may give space for two kids to clean their teeth at the same time in a pinch, and catch more splashes from washing hands under running water. Teenagers rarely put the plug in, so what extra functionality are you seeking by changing the basin? FWIW the taps don’t look centred under the mirror.