Citadel vs Google offer advice

1. Google is an awesome place to work overall (work/life, environment, future opportunities). There is also something to be said for being a front-office employee, which software developers are at Google (not so at Citadel I assume). You will probably get more opportunities and be treated better as the money center at Goog than as part of the back office at Citadel
2. Yes it probably will be, but the same is true at Google. Its very hard to get past the $500K/ yr all in level at a place like GOOG - i'm less sure about where you'd stall out as a software engineer at Citadel, but I'd assume its comparable
3. Generic answer is YES. Need someone better versed to say exactly how big. Per 2, if you're not in a risk taking role I think you're somewhat bounded (won't get the multi million dollar bonuses that draw ppl to hedge funds).
4. Certainly, but you could also do this from Google. Either way the onus would be on you to demonstrate the necessary skills. It would be easier to transition from Citadel because of the possibility of internal transfer (i.e. just much easier to get your resume in front of someone if already at Citadel) and finance experience on your resume (easier to get an interview). Either way you could probably get interviews though - whether you could get through them depends on you putting in the work
5. No idea how reputable Citadel is in particular, but I know the culture of software engineering recruiting is flexible enough that people will give you interviews (they're just looking for the smartest person). I would consider the transition to a full on quant role (if you decided to take that path) much more of a challenge than getting a software development job at another reputable hedge fund or startup/ big silicon valley type firm. Once again depends on you having a strong skill set

I would say you're pretty set either way. Contingent on clearer answers as to your comp potential at Citadel, I'd probably lean Google b/c of the front-office/ back-office difference.