1) Not necessarily. There are situations where there are two clear phases, or even no clear phases (in really weird phase pushes) in a row. And sometimes the tanks have to make snap decisions whether to gather or clear (and overcharge), especially around phase changes or when Rolling Thunder is out.
2) There may be no written rule...but since those orbs are what keeps the tanks safe having a tank let the rest know its OK to clear after they're buffed is a VERY good idea.
3) Lol. Yes, you can do any fight with no voice. But doing so, especially in fights that require a lot of communication, is generally a bad thing. If you have the option for voice, even just listening in is better than assuming you're too good for it
For the record, I've mostly tanked Ramuh EX (prob 20-30 or more kills) and have also DPSed it several times. Tanking is FAR easier than DPSing this fight, since you spend half the fight doing very little, and after getting orbs and taking hate...you still don't do much. Its not a very tanking-intensive fight at all (heck, I rarely use CDs until Rolling Thunder goes out or something goes wrong, and one of my healers is DPSing a lot before then too, or standing doing nothing). DPS have to share half the land with 6 others and make sure to not trip over a stray orb during a gather phase, break (or avoid) the Chaosed people, don't overlap the AoEs, watch for Rolling in phase 3...and know what the tanks are up to
Should DPS be paying attention to the tanks? Yes. But...to say they need to know exactly what the tanks are doing without being told is just dumb. Heck, there's times where I will make the decision to overcharge and yell at the other tank to save the orbs for Rolling or just call for a clear (usually when the system gets screwy if the phase change is off, or a tank died or accidentally overcharged earlier)
Ideally, the clear/gather will always be like that. It rarely happens that way, so having good communication is what saves the fight.