Welcome litsl,
I'm totally biased, but I think you're doing exactly the right thing doing the VdlP in winter. I'm a couple of days shy of the half way point near Fuenterroble, so sadly we won't meet, but my answers to your questions are (but bear in mind that other people will have different answers):
1. Alison Raju worked for me last time, but I'm using the Lepere (French) one this, as it has (out-of-date, as they don't include the A66) maps - it's not brilliant. It would be good if there was an up-to-date and accurate guide - and perhaps there is: check the rest of this forum. But the arrows are the best map.
2. I've not used a tent (or indeed a sleeping bag, just a liner).
3. There is a fountain in every village, and many mountain streams in between as well. There are sections where water is scarce, but in winter temperatures you don't need that much - I was getting through up to 8 litres a day in Andalucia when it was in the high 80s, but 2-4 seems fine in the cool of Extremadura. I've never worried about treating it (mostly, the village fountains are delicious spring water, although for some reason Galisteo was horribly chlorinated). Some of the fountains in Galicia say they are not drinkable but I ignored that and suffered no consequences.
4. Not sure, my merrells come just up to my ankles and are fine. There is mud about (and will probably be more when you get here) so perhaps something above the ankles would be better - but you're just as likely to go in up to your calves as to your ankles, so it may not make much difference).
5. 25 miles a day is fine - I've been doing a bit less this time, and taking deliberate days off, as when I first did it I went by so fast I missed out on seeing some of the most beautiful and wonderful places in the world - Zafra, Mérida, Cáceres, Galisteo, Salamanca, Zamora, and many of the "one horse towns" in between have something worth seeing. My advice is don't rush, but everybody finds their own pace.
6. There's a bar with food in virtually every village, and a mini-mercado in most. You need to plan, as there can be a 20 mile gap to the next village, and some places close on Sundays (but usually not the bars), but if you have some emergency rations in your pack you won't go far wrong (I carry a pound of raisins and a chunck of cheese for nibbling when there's nowhere for lunch or just in case - not a balanced diet, but it'll keep your blood sugar up).
Do hope you enjoy it and keep us posted.
Buen camino.