You have done a magnificent job of going through some complicated research and calculations. Absolutely everything you've done so far is correct. The only issue I will address is reinvested dividends.
If you don't know the exact moment the stock originally traded you can use the value at the close of the sesssion.
When you received the dividend you paid the tax on it. So the amount of the dividend is the cost basis. You would have to determine the shares based on the date you acquired the stock. You would then make the adjustment for the splits.
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