you ask hard questions….which is why I like you.
The water source is (not surprisingly) a model I already own. It’s, I guess, a sinkhole in an arid expanse, filled with pure, cooling water (or resin, to be more accurate). So why isn’t it walled, guarded, poisoned? My guess is it’s sacred (falling back on the explanation for anything an archaeologist doesn’t really understand). So, the NKE deem that if you reach the water, so must the gods want you to reach it. Inshallah! (wrong language but correct meaning).
The Mycs have 3 units of chariotry (the NKE have 5). The Horse are just a quirky extra unit & none too formidable. As you realise, the Egyptians are, overall, more nippy than their proto-Greek enemy, who have some Heavy infantry as well as the usual Mediums, some archers, skirmishers etc.. This means the battlefield will favour the NKE as you point out. It will be largely flat, desert expanse, with some soft Sand, some Rough Going & a few palm thickets to provide some challenge.
If I was the Egyptian commander, I would mass the archers on some gentle rise, support them with the Close Fighters as protection & shoot up the invaders; sending in the Sherdan & chariotry for the coup d’Grace. I daresay the Mycs may have something to say about that.
Have you noticed the similarity between my battle & the real Battle of Hattin – Saracens against invading Crusaders?