First, Tywin’s army was probably nowhere near 20,000 by the time of Tywin’s death. He’d taken casualties at the Green Fork, the retreat from the Green Fork, some casualties from raiding and counter-raiding, heavy casualties at the Fords, and some casualties at the Battle of Blackwater. I would estimate that of that 20,000 perhaps 12-15,000 were left. 

Of those, most left when Tywin died. 

It’s hard to say, as we haven’t seen any examples to rule it in or out. But I would guess that, for any family that would care enough about that status to get married after the fact, they’d probably get a decree regardless. 

Ser Maynard (Bloodraven) points that out, but Ser Kyle assumes that either Bloodraven or Maekar would have them bumped off. 

I don’t think they’re part of any Wardenship. 

Well, it would have to be more than one, but yes, probably. Minor Houses can repopulate, the Watch hasn’t been able to. 

I may do that later. 

That’s a really interesting question. 

The first major change is the formation of a royal powerbloc in Westerosi politics – with the Baratheons, Tullys, Redwynes, Tyrells, and whoever else Rhaelle would marry in his corner, plus his Martell relatives, Aegon V would have almost the whole of the South in his corner. This is more sway than many Targaryen kings have had in a  long time.

Two things flow from this immediately: the first is that Aegon’s Reforms (god only knows what they were) are going to go a lot further and get implemented across a broad swathe of Westeros (which makes the holdouts politically isolated), they’re going to last a lot longer (and he’s going to have a lot of tools to deal with the blowback (armies, navies, money, votes in a  Great Council, etc.). The second thing is that Summerhall doesn’t happen, which means the Targaryen dynasty as a whole is going to be a lot more secure, with three princes hopefully siring kids. 

The more long-range possibility is that big events like the Rains of Castamere or the Southron Ambitions conspiracy might get butterflied away entirely – a stronger Aegon V might be able to permanently put a lid on the Reynes short of outright destruction, and without the Baratheons and Tullys, there isn’t much there there to the SAs. 

My reading of the text was that she seduced him. 

The Dothraki Sea doesn’t really have strategic points or river crossings, that’s kind of the point. 

I’d say what you need is:

1. Combined arms with disciplined infantry and cavalry. Archers and pikemen working together could hold off the lancers and the screamers while trading evenly with the horse archers, especially if you can train them to form square (multiple squares that can mutually support and pour fire on the Dothraki from the flanks and rear is a good idea). Put some good heavy cavalry in the center and use them to counter-charge vs. screamers and lancers, make sure to train them to regroup rather than pursue (keep them near the squares at all time), and whenever possible to wheel and charge the Dothraki towards a square to catch them on the pikes and prevent feigned retreats.
2. Mobile fortifications. I would use a combination of the corral method used by the Boers and the Roman fort-building, together with the liberal use of entrenching tools to construct ditches and berms to reshape the battlefield. The Dothraki thrive on being able to attack on horse from any direction – with mobile fortifications, you can deny areas of the battlefield to them and make yourself much harder to attack from horseback. A dismounted Dothraki is much easier to kill.
3. Lots of supplies and firepower. The real danger is that you beat a handful of khalasars, but run out of food/water or arrows halfway across the plain and then you get swarmed and die. So you want to bring a lot of supplies in your caravan – again, using corral techniques to protect your wagons. 
4. Utter ruthlessness. This is the part that really troubles me. Because if you were going to take the Dothraki Sea, you’re going to have to do some very ugly things. Murder their herds of horses, set grassfires from behind firebreaks etc. But the worst is the deliberate destruction of Vaes Dothrak to destroy their morale and sense of manifest destiny. 

Pretty damn well. After all, Garth the Greenhand outdid Aegon IV when it came to sleeping with every woman he could, and he’s revered in the Reach. As long as the King is good at their job/lucky enough to be ruling in a prosperous and peaceful time, the epic-manwhoring feeds into the whole Fisher King/fertility god thing and the people will think you’re a virile, merry king who’s responsible for the good times. 

It’s when you fuck up or things go badly that they start to say you only think with your balls.