Varios questions

Go by train. Driving and parking in unfamiliar cities can be a real drag. The trains take you city center to city center. It is almost always a better deal than driving. I only recommend renting a car if you want to get away to the mountains or somewhere where regular train or bus service doesn't go.

Yes, highways are clearly marked with international signs. This is Europe, btw. Not as easy to find your way in the city though; highway and freeway access signs can be confusing or non-existent.

Sorry, don't have advice on the other topics.

I never know what affordable means to people. Could you give us a clue?

Hi

Is there any reason you want to stay overnight in Toledo and Aranjuez? You can do these as day trips from Madrid. In Madrid we stayed at the Marlasca Hostal - www.hostalmarlasca.net. Excellent location, near Puerta del Sol. Again what do you mean by "affordable". In this price bracket there is certainly no shortage of excellent accomodation. Please note a hostal is not the same as a hostel. It's sort of like a B&B without the breakfast. Happy travels.

Here is the link for information about the airport at SdC. Here you will find all the airlines that fly in and out. Then you will have to Google for the company.

SdC airport information

We would not advise you to hire a car if you only want to drive around the cities. In fact with SdC you cannot drive a car around the historical part.
Prices of the car will depend on your requirements, you age etc. Choose a major company if you want to hire in one city and return in another. For companies again use Google.
We note that you are here for only one week. And you want to see all of those places. That is almost one place per night. You will be travelling all the time and not seeing the cities.

Cheap flights to Santiago from Madrid with Vueling (www.vueling.es) even though you can get good offers on Iberia (www.iberia.es) from time to time. Try www.rumbo.es to explore prices from all four airlines that program this route on a daily basis.

You'll have a great time in Santiago. Nice city, good food and fancy places to go out.

Having lived in Aranjuez, I would not recommend spending the night...there's just not enough to see. Perhaps during canival would be the only exception to that. Toledo, on the other hand, it lovely, and worth spending at least one night in.

You only have one week, and you want to stay in all these places? I would suggest staying in Madrid, and see Toledo as a daytrip. Only if you feel you have time, go to Aranjuez (not on a monday), but I have to agree with Cairogal, that besides the Palace and the surrounding area, there is not a lot to see. I would rather suggest Segovia, as a daytrip from Madrid.

For planes to and from Santiago take a look here: flightassistent., and remember what flies one way, also goes the other way. This website only list the budgetairlines, but you should also consider Iberia, as for national flights, they can give you prices, that are just as good as budgetairlines. (not always, thou)