Buying 0 or 2+ copies of Giant seems to increase the chance you will play it an even number of times, which is good. Right?
I don't think this is right... with 2 Giants, I'm not sure that your chances of playing it an even number of times increases over just 1 Giant. Let's say for 1 Giant, there's a 50% chance of playing it an even number of times... this should be about true on average. For 2 Giants, each one still has that 50% chance of being played an even number of times. So there are 4 options, each with a 25% chance of occurring:
Giant 1 played odd #, Giant 2 played odd # - Even number of Giants played
Giant 1 played even #, Giant 2 played even # - Even number of Giants played
Giant 1 played odd #, Giant 2 played even # - Odd number of Giants played
Giant 1 played even #, Giant 2 played odd # - Odd number of Giants played
Still comes out to 50/50.
Now if you're drawing your entire deck each turn, then this changes; because you won't ever end the game while your Giant hasn't been found in the current shuffle. In that case, 2 Giants would mean you always play even; 3 would mean you have a 50/50 chance of playing even or odd.