Would you please answers my queries?
1) He must have been completely sober, as he had drunk so little alcohol.
(How to justify the use of so here, is it possible to use very for example?)
2) a) It was a rather / fairly good result but you could've done better.
b) We had a rather / fairly nice meal last night.
(which one is best and why? What's the difference between "fairly, quite and rather"?)
1- You could use 'very'- that's the sense.
2- Little difference, but 'a rather good meal' sound better to me.
Differences- you can use 'rather' before a comparative and 'quite' with a superlative. Also, 'quite' has the extreme meaning (quite brilliant), which the others don't have.