"So the stock price will drop $1.50 after dividend?"
Somewhere in that range. Contrary to what many seem to believe, the opening price of a stock on the ex-dividend date is not automatically reduced by the amount of the dividend. A stock is free to trade at any price at the open, but considering that the dividend is no longer available to buyers on the ex-date, traders are usually unwilling to pay the same price they would the day before.
"any future dividends will not be "pro-rated" if by this you mean that if any future dividends will be adjusted for the passage of time between normal dividend dates"
"If the sale closes, say 2/3 of the normal time for the quarterly dividend, I suggest that we will get nothing"
Your suggestion is in error.
It is very common that dividends are prorated in this kind of situation and this will have one too. Instead of speculating, you could've kept current with company news and noticed that the company announced a prorated dividend six days ago: " the board of directors of TECO Energy today declared a special pro-rated dividend at the current rate until the closing of the merger. ... The board of directors declared a dividend of $0.002527 per share per day that will accrue from May 16, 2016 (the prior TECO Energy dividend record date), until and including the day prior to the effective day of the merger or until the next regularly scheduled record date of Aug. 15, 2016, whichever occurs first. This dividend is the daily equivalent of the current quarterly dividend rate of $0.23 per share."