We often go Costa Brava at easter (top of Spain, Mediterranean coast) -
A) Varies year to year & day to day, would expect temps to be 16-22 degrees maybe one or two days of rain. However it does get cold at night.
B) From what we've seen, not a lot open. To be honest we didn't try too hard looking for open campsites once we found we could get a superb villa at reasonable rates, we normally pay something like £40/ night for something typically with 4+ bedrooms, 2+ bathrooms, heated private pool, WiFi etc. We then just use the Cali as a day van & something to sleep in on the journey there & back.
C) I don't know but we've not had problems with Mozzies that early in the year.
Our favourite location is Tamariu & have found there that compared to the summer many restaurants & shops are not open during the week only weekends. In the bigger towns i.e. Pallafrugal almost everything is open.
We've sat on the beach in shorts & T shirts in 20 degree heat at the end of March whilst the locals still have fur coats scarfs & gloves on looking at us as if we are completely mad.