Q1: Can oil change be done at 280km of running?
Ofcourse oil can be changed any time..It is recommended to change oil every 500kms during break in period.You can change your oil at 300 then 800 and after that every 1000 kms on local oils.For rough road conditions as the bike undergoes continuous gear shifts.It puts an increased toll on engine and thus the lubricating oil.1k kms oil change is recommended.
If air filter is foam type clean it after every 1000 kms with kerosene oil.If its conventional replaceable filter replace it at every oil change.
Q2: Is it necessary that the bike to be tuned after the first thousand kilometers or can it be done after 2~3 hundred kilometers?
It is always recommended to follow manufactured guidelines regarding service intervals.Recurring tuneups for a mere 300kms interval is NOT AT ALL recommended.Bhut ziada tuning karanay say bhi bike ki performance effect hoti hai.
Q3: How frequent the oil change should be done ?
Every 1000kms or 3 months whichever comes first.Minimum 1000 max 1500.
Q4: And how often the bike should be tuned to keep it perfectly working?
Cg-125 manual states every 2000kms(maybe).My manual recommends tuning after every 2k kms..FOr 70cc bikes interval is 1000kms.
For rough usage you can tone down tuneups till 1500kms
Q5: what parts are checked by the mechanic during tuning?
Oil change(if required),Tappet adjustment,chain adjustment,brake/clutch adjustment,re-lubing parts,tightening nuts and bolts.
DO NOT USE USED OIL FOR LUBRICATING MOTORCYCLE PARTS.Buy a bottle of gear oil and order the mechanic to pour in fresh oil on chain and parts.