Which means they couldn't maneuver at all at the attack site, making themselves extremely vulnerable; Furthermore, a dam is an highly fortified target by nature, making serious damage requires very heavy bombs, not sure they could make it even as a one way trip if they are properly armed.

IMHO, most likely result of an attack (for Egypt) is that Egypt loses a part of its airforce in return for temporarily harming Ethiopia's ability to generate electricity at the site. At which point Ethiopia fills the dam anyway and works on repairing the damage, while Egypt wouldn't be able to repeat this.