Rabadon's Deathcap is meant to replace Void Staff if:
a. the match is continuing on extremely long and you acquire the money for it.
b. you are getting somewhat fed, or you can afford at least a Needlessly Large Rod in one trip back to base.
Please also note that if you get Rabadon's Deathcap due to option (b) then you probably wont want to replace Sorcerer's Shoes, as the only reason that get's replaced is because Void Staff's passive takes into account the penetration that already exists before being applied.

After the July 3rd patch, Orianna's skillset is more based on burst damage with her W, support with her E, and ball placement with her Q. This means W is going to be the primary source of damage now, which is why it has been changed to be maxed first. After maxing your W, you will want to max E because of the second half of the description. It deals some damage while the ball is in motion if it hits an enemy. That in itself is more or less a good reason to not max Q. However, using your E to deal damage as well as protect takes a little bit of practice and well thought out timing, so if you are not having luck with it, i recommend alternating between E and Q upgrades, with Q taking first priority (so you would put a point into Q, then E, then Q, etc.). Q does have a shorter cooldown as well, which may lead to an overall damage increase, but also keep in mind that Orianna's shield is very useful for early game sustain and really does matter. If you're doing good then by all means, max Q first, but if you find yourself needing a bit more sustain, I recommend maxing E.