I would not have minded extending the movie a few more extra minutes observing Steve returning the stones.
Red Skull seems to pop up unexpectedly whenever he receives guests, so I would say Steve sees him. That would have been a VERY interesting conversation worthy of the movie in my opinion.
Guardians of the Galaxy Volume 3: The Re-Kindling of Romance. That title would make Rocket barf.
How did the rules change? The Soul Stone appears only after one person's loved one is sacrificed. Natasha and Clint did not love each other romantically but they did love each other.
Natasha loved Clint so much she wanted him to regain his soul. The only way was for Clint's family to return to him. The choice of sacrificing herself was hard but easy at the same time. Extremely sad but... instant Soul Stone.
Is the question about the rules have anything to do with the timing of Natasha and Gamora (the original) deaths that resulted in the Soul Stone re-appearing?