my opinion on #4.
I recently had to figure something similar to this out, so I thought I would share.
Different Breweries / Brewers use different "Growth Factors". In searching the internet i've come across numbers from 3x to 10x. I see alot of mention of a 5x Growth factor.
White Lab vials contain 75-150Billion cells. So if you average that and say 100Billion cells.
The largest starter would be one that allowed you to go 100B x (10x) growth factor or 1 Trillion cells.
Using YeastCalc that works out to ~44Liters of starter or 11.6gallons.
So the simple answer would the max starter for a single vial would be about 11.6gallons.
However notice that if you follow the common growth factor of 5x you would end up somewhere around 5-6 gallons. Hence that is why White Labs and Wyeast pack that specific amount of yeast in the packs.
The higher the growth factor the higher your chances of altering the metabolism of your yeast. The lower the growth factor the smaller amount of cells will be grown.
But what if you make multiple steps? Then you will grow more and more cells.
The drawback however is every step you take increases the risk of contamination.