They won't go to worthless. After all, Tesla is a car company and it makes a fair number of cars. It does have some intrinsic value.
But it does mean that Musk's collateral will decrease in value which is the kind of feedback loop he probably wants to avoid.
Well they are clearly not the same thing. In a margin call on a loan, the debtor has two options. They can either increase their collateral or they can reduce the size of the loan. The latter only involves selling shares if there is no other way to raise the money.
The huge sell off in the tech sector is already happening. Look at what has happened to Meta's stock since the beginning of the year from over £300 to just over $100. This is the main reason why Musk was trying to get out of the Twitter deal: Tesla stock was on its way down anyway (actually, Tesla is not a tech company any more than any other car manufacturer, but it is perceived as such).