1) I would like to understand why altcoins are traded in Bitcoin and not directly in dollar ?

because the demand is for bitcoin not for dollar...
in other words people are trying to directly increase their BTC and also because trading with a cryptocurrency (deposits withdrawals, AML/KYC) and all that are easier.
it's really a problem that almost all of altcoins depend on Bitcoin price because when Bitcoin price drop, altcoins drop too and conversely (that's what happening right now) ...while if altcoins were traded in dollar, each crypto were independent...
If tomorrow Bitcoin totally crash , altcoins will crash too...
they don't depend on bitcoin price, altcoins are a side product so they can not function alone. when bitcoin has a drop they drop as a side product.
also it is because traders are still interested in bitcoin not altcoins. when bitcoin drops they take their money out of altcoins and put it in bitcoin market. it can be buying the dip or shorting bitcoin while it is dropping. but in any case alts drop hard because of it.
2) What are your tricks to keep monney when bitcoin pump and dump ?
stop holding altcoins so you don't lose money.
only trade altcoins while they are going up. in your example trade it when it is the time to go to $2.
check the history of altcoins, almost all of them rise in
a couple of days.