I just walked from Sevilla this past May and June. Some days in the south and strangely later in Galicia were extremely hot...in the south +45C. In my experience, particularly in the south there were not many places available to find water or a bar during the day and so I carried water from where I started, this improves further north and for sure in Galicia. The most I ever carried was 2.5 litres and on that particular day I drank every drop of it (third stage). I use a hydration system and most days I would carry about 1.5 litres with me plus food for the day...sometimes more, sometimes less depending on the forecaste temperature and elevation. Using the hydration system helped to keep the water at an acceptable temperature in my pack so potability in the heat was never an issue.
One of the best and most obvious ways to beat the heat is to indeed get up early and walk in the cool of the morning, starting in the dark if need be. The Germans, Italians, French and Spanish are going to do this anyway and they WILL wake you up, so you may as well get up and join them
I did, and I never got lost...just take a moment each afternoon to make sure you are clear about how to get out of town in the morning as its not always very obvious. Once you are underway its usually pretty straight forward (but not always). Many Europeans start off without head lights or a flash light and just walk...I thought this was not the best idea and used my head light and a very light, cheap small light (one of those bike lights with a steady or flashing light combo you put on your back) on my front chest strap whenever I was on a road in the early morning. As you know, the idea is to finish each stage before the worst heat of the day which typically happens after 1:30 or 2pm.
The VdlP is an amazing journey with some long stages full of beautiful scenery. Honestly, simply plan your gear well and then just take each day one at a time and things as they come. No matter what you plan in the way of stages before you leave, you can count on making lots of changes along the way as you fall into the rhythm of the journey and the people you meet along the way. When you meet interesting people you tend to match stages for a while so you can get to know them in the evenings after walking alone all day. I loved the Frances when I walked it last Sept/October, but for me the Plata was better in so many ways...it just depends on what you are looking for. And yes, there is Aquarius
If you would like more information please let me know or send me a PM and I would be pleased to share. Buen Camino!