Pokes
Infinite Lives
POKE 72BF,00
This removes the command;
72BF DEC (HL)
Which is used to decrease the player life counter held at 66F0.
Aliens Don't Fire
POKE 7B67,C9
Just RETurn immediately from the alien subroutine.
7B67 RET
Mothership Alien Doesn't Fire
POKE 7B67,C9
Just RETurn immediately from the mothership alien bullet subroutine.
6A9D RET
Immunity
POKE 6CB8,00
6CB7 LD A,01
6CB9 LD (66A4),A