IIRC The breath isn't consumed over time, it's used up instantaniously at the end of the week, it neer says this in the book as far as I know but it's the only way that would have allowed Vasher to survive with out breath, as long as he had breath in him before it would be consumed he would be fine with no breath in him at all for the rest of the time.
1b: It hasn't been going on for very long, only as long as Hallandren has been separate from Idrian (sp?) I don't remember if Siri's father was the king that they had seperated from or if it has been a king before him. if Siri's father was the king then the Returned haven't bee sustained for more than a few decades, if it was a king before him I'd be willing to bet that it's been less then a 100 years since they seperated. I dont remember how many "gods" were in the court of gods but I'd be willing to wager it couldn't have been much more than 20, if a year on Nalthis is similar in length to a year on earth than that gives them roughly 45 breaths they need a year, (weeks in the cosmere are 8 days IIRC) 20*45=900 add another 365 for the God King (he gets one a day, right?) and we end up with roughly 1265 breaths a year to sustain the court of gods, some years it might have been more and others it might have been less. if Hallanderan has been arround for nearly a century then that puts it at roughly 1,265,000 126,500 (a century is NOT 100 years don't know why I multiplied by 1000) breaths used on the court of gods, I don't find this number to be very unreasonable what with all the children that give up breath.
It has been a while since I've read Warbreaker so someone else might be able to better answer you other questions
Edit- Fix math error
Edit2- Add reasons for edits.