1) WC ventilation
I ask my fren how did those ceiling type WC ventilation/exhaust fan works, he said need ducting so the air/whatever can go outdoor ..
then one of the contractor told me no need ducting, cuz it only suck nothing else but hot air ...
doesn't sounds logical, i thut the ventilation fan oso suck air, moisture, bacterias (maybe) and etc ...
if no ducting then all this thing will be deposit to my root lor?

If extracting into the ceiling then no need ducting. It's just blowing hot air into the roof, and pulling in cool air from the surroundings.

2) shower
Was planning to install solar heater ..but stepped back due to the cost (heater itself + cost per point + tiles) .. so I go for individual heater,
but again the contractor told me if use individual heather then the original shower/shower head will rendered useless,
cuz normally heater got it own shower head ... i'm thinking individual heather + original shower head ...
does that again required hacking wall, re-do piping, tiling? Is there any other solution?

Yes, you need to use the shower head that comes with the individual water heater. Original shower head in the bathroom just remove and seal the pipe lor (low cost solution) if not need to hack the wall to remove the pipe outlet and then plaster or tile it back.