A Gravemind can corrupt AI without using computer code whatsoever. In that department it is more effective than what the Reapers tried in ME3. All it needs to do is establish communication and it can convince the AI to go rogue.
Well if it's his space-magic power to just corrupt AI's okay, the Geth won't work.
If the change is similar to the subroutine alteration stored by the ME2 heretics (and this interpretation is supported by canon), that won't work. It will simply propagate as the geth interact with each other. Again, this is way scarier than what the Reapers conjured up. Changes to basic logical processes can be enacted without direct alteration to code. One of the Graveminds was able to turn Mendicant Bias rampant through conversation alone.
Again, if it's space-magic, alright then. If it's a change to their basic programing, then again it would still have to affect them all simultaneously (or individually post-ME-3). Geth share information, but do not necessarily share opinion, as evidenced by the existence of two factions.
If synthesis involves the combination of organic and synthetic qualities, then they can be infected. Any defenders would have to be completely non-organic.
We don't really know. The consequences aren't well established in the Mass Effect universe, and entirely foreign to the HALO universe.
A Gravemind's intelligence and computational power grows exponentially with mass. It can create and coordinate strategies on a galactic scale and create dopplegangers from the ecosystems of entire planets. There are an estimated 20,000 Reaper capital ships and an ungodly number of destroyers. The Reapers are almost certain to have more collective computing power, unless it's the square of all flood mass in existence in a set radius.
And the Flood isn't a virus. Its morphological capabilities far outstrip anything a Vorcha's biological system can muster.
Salient point.
Although I still feel that reaperization would offer a viable means of resistance. Subjects are likely not truly living, nor do they appear to be fully sentient, or even partially sentient in some cases.
Granted it would be a measure of last resort. But as 343 Guilty Spark put it, protocol dictates action.