Honestly, I don't know exactly how many liters of inert gas you would need (I'll let you figure out how many liters are in the bottles you intend to use, thus how many bottles you need). I don't know how to crunch the numbers for that situation.
Again, the math that appears here several times: with the typical protocol, 40 minutes to death x 15 Lpm flow of gas = 600L of inert gas.
You can turn to the All Knowing Wikipedia:
https://en.wikipedia.org/wiki/Lung_volumes if you want to try and crunch numbers, but there's a lot of complex math vs. physiology that isn't shown on the Wiki page that I just don't know how to calculate.
However, after thinking about it for a while, the big problem that comes to mind is timing when you pull the bag down relative to when the sedatives kick in. I don't have any experience with sedatives, but judging accurately when you're close to slipping under, and while you still have coordination turning on the gas, filling the bag (it takes several minutes), and making ready to pull it into place, all timed so that you have minimal time between bag-ready and bag-down (all that time between bag-ready and bag-down, you're using gas at 15 Lpm), seems tricky.
I think you would need
a lot of inert gas to give yourself enough time, while still coordinated, to do the last preparation, and then wait further until about to pass out to pull down the bag.
If I had to guess how to accomodate that "sedated eb" protocol, I would probably double the typical amount of gas, to have any confidence at all.
As for what to do if it's not pure He --you won't know it isn't pure until you don't die. I have no idea what the risks are of brain damage in a low-O2 atmosphere, but I'd be surprised if there wasn't at least some risk.