1) They can't. They have to use total population to apportion districts. If they tried to use only the citizen population, it'd get to court so fast their heads would spin. SCOTUS ruled as recently as 2016 that non-citizen population must be included in legislative district totals. That part is a bluff. Including the question itself was always just a play to depress Hispanics' response rates. They still would've had to count non-citizens (people who answered no to the question) in the totals. They were just hoping people wouldn't respond at all.
Can't they just draw the lines however they want though? Look at the citizen data, draw the lines so it's most advantageous to them, and say it's a partisan gerrymander.