Hot Water Problem

Hi,
New to the forum.
Couple of questions...........on my Weil-McLain Gold Oil Burner
1) I have noticed that my shower water requires a bigger turn of the faucet to get hot water. ......... what is the control on the boiler to turn the water temp up? and.......... why would it change like that? (boiler was serviced in December).
2) My boiler has been discharging (through outlet drain pipe) quite a bit of water ......... maybe about 1 gallon every couple of months........ is this normal? ..... if not, then what would cause that?
3) A couple of years back, my heat registers were making noise.... believe was told by our oil company (who also yearly service our system), that would cost about $300 to remove air from the pipes? Is this something that need professional to do? Also.. have read that coils may need to be cleaned????

Thanks for the help.
Bob