1. Wouldnt worry, it will wear and slowly get looser, check for the five cent test.
2. Depends on the bean grind,freshness,roast level etc when it is about right you should be able to get the 20-25ml
3. I would stick with the double basket and just run a single thru, no complaints from my wife who definitely lets me know if I run a double thru.
Also I wouldnt get to hung up on pulling a shot to the second or ml. Does it taste alright? are you happy to make coffee for other people? then its about right!
p.s. I find coffees that i am not that happy with most guests (non csers) will rate them better than the petrol station ones they have been drinking.