What is divestment but a form of boycott?
Boycotts of a company's products work because the operations of a company are making and selling the product, and not buying that product cuts off their income, lowering their profits. Boycotts of a company's stock, on the other hand, have no affect on the company's operations at all, and do not affect either income or profits. The business goes on as normal.
There is _nothing_ that compels an "automatic, equal and opposite transaction"
Um, yes, there is. The fact that until someone buys the stock, you have not actually sold it. This is really, really basic. Otherwise there's no sale. Every single successful sale of stock ever, by definition, is also a successful purchase of stock by the counterparty. Until someone other than Harvard buys the stock from Harvard, Harvard has not yet divested itself. Since the amount the other party paid and the number of shares they receive is identical to the amount Harvard receives and the number of shares Harvard gets rid of, the divestment is automatically coupled to an equal and opposite investment.
selling stock can actually generate a positive feedback loop that causes _more_ stock to be sold.
But it cannot cause more stock to be sold than is bought. Because without a buyer, there is no sale. Transactions have two parties and two sides and both sides are always in balance.
Yes, if lots of people want to sell the stock and few want to buy, that makes the stock price go down, and because lots of stock traders are stock price speculators, that can have a herd effect. So what? The price of stock does not affect the operations or the balance sheet of the business in the slightest.
Stock price only matters to investors who are gambling on share price appreciation. There are a lot of investors like that, but not all. Plenty of people buy stock for the dividends, and your divestment campaign simply makes the stock more attractive to those people. Net, what you do with a divestment campaign is change the owners of the company from people who want the stock price to go up to people who want the company to pay high dividends - and the former care a lot more about bad PR than the latter. Divestment, insofar as it has any effect on a coal company, will make that company less sensitive to social pressure that might affect the stock price and more interested in maximizing operational profit to maximize dividends.