I still find IV V helpful for anyone and much more powerful than oral.
To answer these questions:
1)yes, mason jars are great improvised containers for stock solution. However, boiling water may not be enough to sterilize. A pressure cooker is better.
2) it's not reccomended to add the vitamin c to saline because then the solution will become hypertonic.
3) again, this is another way that you can use a larger size mason jar for. just use a larger quart size jar, drill a hole in the lid, buy a self-healing injection port from a mycology supply store, fill with purified water, and then sterilize in a makeshift autoclave (pressure cooker). Plug an IV line into the injection port. wrap the mason jar in ductape and attach a rope handle and hang from a hook on the ceiling.
4) this is irrelevant. you don't need anything to stir with, just put the lid on and shake it up.