i dont know what the perks of doing on wheel balancing is. to my knowledge on wheel balancing will damage the gearbox due to the absence of lubrication, because the oil pump is not running when u do on wheel balancing. i always opt for off wheel balancing.

normal balancing and allignment is good enough for most cars. Your Pug 308 THP is no biggie. try these places; GS Auto along Jalan 222 or Sunny Car Care in Taman Sea (look for Ah Wah). I normally recommend Audi and Bentley drivers to go there.

your problems could be due to:

1) use of different tyres
2) damaged susp arms
3) tyre pressure differences
4) camber allignment

i hope you know what you're talking about. brake rotors cant be balanced. if its worn out its replaced or skimmed if its warped. and what assembly are u talking about?
as for the gearbox the oil goes back to the oil sump at the bottom.