I don’t think you’re taking into account defense mutlipliers with at least 20,000 men in the Tyrell army inside the walls being able to take on ten times that number, and the numbers you give are around 50,000. Ships could block the passage across the Blackwater. They could hold off Aegon long enough for a relief force to arrive from the Reach or the Westerlands. There is still the problem of another Tyrell army in the field.   
Garlan’s not coming to help? If they lose HG they can retake it, but if KL falls they lose the war. Besides, Redwyne’s fleet has arrived to keep the Ironborn busy. If Tywin Lannister was willing to forgo aiding the Westerlands to save KL then I think Garlan would do the same.
Unless you have copy of TWoW in front of you, I don’t think you can effectively say that Aegon won’t go to Oldtown anymore than I can say he’ll go North (I don’t think he will). The Hightowers are the most powerful vassals of the Tyrells, and as Stannis demonstrated, aiding lords against Ironborn is a good way to win support. The Hightowers are pissed with the government in KL over their lack of response to the Ironborn. Winning them by aiding against Euron when assaults the city would be a massive blow to the Tyrells. If Aegon plans to win Reach lords he may have to go into the Reach, Aegon is going to need every sword he can get to defy the power of both HG and CR. The Hightowers were also the chief supporters of Aegon II in the second Dance.   
The HS crowned Aegon I in Oldtown, and none of the other claimants waited to be crowned in KL except Joffrey. There is precedent for being crowned in Oldtown as mentioned. There is nothing stopping the HS from going to Aegon. 
Except Aegon cast her down in that interpretation when it is supposed to be the younger queen. Whether or not Dany arrives, Cersei will have been dethroned and lost all she holds dear: her kids, her seat and her power. That still begs the question of why the prophecy mentions Dany if it doesn’t fit in any way?
I think we will wait and see in TWoW. If I am right I get exclusive bragging rights, the same goes for you. 

Mace is going to have to take to the field against Aegon – so no defensive multiplier.

You have it backward – if HG falls, the Tyrells fall. KL can be retaken. Look again at your Tywin example. When it came down to it, he chose to march west rather than south, because losing Casterly Rock would have lost him the war. These are feudal armies that exist as long as the lords and knights who make them up choose to follow – once you’re seen as weak, your army crumbles.

Going to Oldtown goes against all military logic, and there’s no reason to do it except for an over-literal replaying of the Dance of the Dragons. Aegon I was crowned in Oldtown, but every King since has been crowned in King’s Landing, because that’s where the Iron Throne is. There’s a reason why both Stannis and Renly went for King’s Landing and not Tywin – because King’s Landing is where political legitimacy comes from. 

It’s going to fit – Cersei won’t have fallen completely yet before Dany arrives to finish the job.