RE:Shorters lose 50 % margin under 5 $...so good level ...
I think you have this backwards, longs lose margin and have margin calls. . Shorts don't own the stock, the margin is generally setup at the short sale. So if you short sold 1000 shares at $20, you would have needed 150% so $30,0000
As the stock plummets to $5, if they add 1000 shares at $5 for a total of $5000, they need a margin much lower so $7500.
It any longs on margin are going to hurt here. So when the SP goes down Longs have to worry about margin calls. When the SP goes up, it's shorts that have to worry about margin.
https://www.investopedia.com/ask/answers/05/shortmarginrequirements.asp