Read the find print, but probably not. Index funds are aware of the issue you raise and they all have plans to handle it. Plans range from "not a problem, ignore", to "we don't even try to have the same stocks as the index, just similar stocks that we think will match the index performance". Most are someplace in between those extremes.

Yes selling will happen, and in the case of the S&P 500, it will be weighted selling across the whole index.

Spacex/Anthropic/OpenAI almost certainly won't crash the market. The most probable thing to happen is that all 3 of these rally a surprising amount on their opening day, because there will be so much forced buying of the shares.

In my opinion, the most likely bagholders will be any retail traders that buy these stocks before the lockups expire.

I think it's very likely that we see the following:

IPO day -> all 3 close higher than opening price.

1 month -> price settles into a range 20-30% higher than IPO price.

6-12 months -> price is back near IPO price +-5%. Anyone who bought and held in the first 3 months has unrealized losses.