But dividend payments are not actually reflected on the actual price-curve. Meaning, if a stock closed at $10 with a $1 dividend, the dividend-adjusted closed is $9, but the actual price of the stock if I want to buy it at 3:59PM EST is still $10, not $9.

It seems to me that if I use a div-adjusted price history, that price curve is NOT real, it is calculated. I only benefit from the div-adj price if I actually hold the trade through dividend date. Otherwise, entry and exist signals that are not reflective of what's actually happening on the tape -- for example, a positie close for the day vs. a negative close (because of dividend adjustment).