Hi again labgirl!
A good friend from Canada ("stanbr" on many of these forums; look him up!!) always takes that Air Canada flight that gets in at 10 AM -- I take a similar flight, USAir from Philadelphia ... these are virtually NEVER late, even if by chance htey're delayed a hour or more takng off from N. America ... Why? because they have all that time over the Atlantic to make up any lost time. By contrast, if one changes in Europe in mid-morning, many opportunities for Delay.
Furthermore, if you plan to fly domestic to Santorini, that is THE island with the most flights per-day. AND if you go to Aegean air website and sign up for their e-mail "newsletter" (really just an alert), you'll get notification of "promotional discount" fares about 4 months before your arrival date -- you may get a flight for as little as ?70E. I would advise allowing 2+ hrs from touchdown to domestic flight departure. In September, it will take u less than 1 hour (maybe 45 minutes) to disembark, claim luggage, grab some Euros (or Drachmas????) from ATMs, and roll your bag to the other end of the SAME building to check in for Aegean/Olympic (they are now the same company. Ryan Air also flys to Santorini but usually at midnight or Dawln NOTE: There is usually a flight somewhere between 12:30 - 14:30 --- BUT ...if you are arriving ATH on a Sat it is harder to get space on these flights, because that's a popular arrival day for folks who must work 5 days a week! Here's a good website to check for all 3 flight schedules. http://reservations.airtickets.com/airtickets/com/search.do?sid=3&xid=1429896126945 ... if you see a flight that works for you, I advise to jump on it -- since u are short on time, you must make up for that with $$. Alas.
Brotherleelove is THE reigning expert on Santorini accommodations, especially in Oia. He's stated very clearly plusses/minusses of all the island locations. Myself, as a budgeteer AND island explorer, after a few stays in Fira, have found Firostephani to be a great "happy medium." NO crowds (because tour groups don't stop there) yet easy 15-minute walk to shops, cafes, nightlife of Fira ... either along the Rim Path, or the more level "hidden" flagstone lane paralleling the main road. There's also a handy bus stop on the main road near the hotels, to get up to Oia, and down to Akrotiri. Some hotels I know about here are Sunset Hotel, Blue Dolphin, Mylos, Gaby Apartments, Agnadema Apts (all w. great views, no pool) ... my own sweet super-budget favorite, Villa Fotini, is just about 75 feet off the rim path, family-run, 12 spotless rooms w balcony, breakfast, free shuttle to ferry, AND pool-use privilege at next-door hotel... what's not to love!?! PLUS a wonderful taverna only steps away, Aktaion, longest-running restaurant in Santorini (why? great traditional cooking, low prices) ... and a sunset view too!