Any chance Aegon could hit Oldtown to repel an Ironborn attack? That is to say, using Stannis’s approach: Winning the realm by saving it, then proceeding to KL; but with Connington sowing a plague all along the way.

Nah. 

1. Saving the realm to win it is meant to distinguish Stannis from the other claiimants. Aegon isn’t a main character, he’s a foil who’s going to die. 

2. Aegon has to be dealt with conclusively and quickly. There’s not time for Dany to have a drawn-out war with Aegon AND save the day when the Others fall AND deal with the stone dragon, etc.