Also, I would be a lot more pessimistic of the index tracking fund managers’ ability or willingness to find extra shares: their goal is to match the index, not beat it. If the index includes the new firm at a blown-up price because everyone sent their buy orders at the same closing auction, then all the index-tracking funds still track their underlying index. They do not care that after that closing auction, the price of the new firm—and likely the index itself—is going to drop.
One thing is fiduciary duty to the shareholders, another is “pleasing the shareholders” as you describe it. Pleasing the shareholders is necessary only when displeasing them means they will sell the stock when there is no buyer. If there is a buyer, the current shareholders are less relevant — as long as management cannot be accused of not fulfilling their fiduciary duty to them.
Also, I would be a lot more pessimistic of the index tracking fund managers’ ability or willingness to find extra shares: their goal is to match the index, not beat it. If the index includes the new firm at a blown-up price because everyone sent their buy orders at the same closing auction, then all the index-tracking funds still track their underlying index. They do not care that after that closing auction, the price of the new firm—and likely the index itself—is going to drop.