Hi Nicholas,
most of your questions are already answered by me and some the members here but I understand that there's too much info to read at once.
I'm afraid there's no way I could tell you which operator has the best coverage in Kerkyra or Symi, since the 3G by it's nature varies significantly in signal coverage and quality, not only in different parts of the same town but even in different spots of the same house. The best way to know which one has good data coverage/quality is to test them on the locations you are visiting (I have detailed how to on my first post).
In general though, Cosmote has the most extended coverage in Greece while Vodafone has the best (=speed and latency) network.
If you decide to purchase a Cosmote SIM and since you will need voice too, you can get a Cosmocarta OR a What's Up SIM Card (not both, there's no need for that). Yes, you just walk in and purchase one of these in any Germanos or Cosmote store but you will need your passport for the registration and the activation. You can read the experiences of wisniak and the DeBilbao on this and the previous page regarding the purchase/registration/activation of a sim card.
After the activation, if you want to have data on your
phone, you can purchase the 60MB (for the moment it's 120MB) data addon for 3€. You can add as many data addons as you like, and each time 60 (now 120MB) will be added to your remaining data volume. However, if you already have the Huawei WiFi modem, you won't need to purchase this "small data" add-on. You can just buy the "Cosmote internet on the go me karta" and connect your Phone wirelessly with the WiFi Modem.
For data on your
notebook/tablet, you need to purchase and register/activate a
second SIM card which does only data (not voice or texts), with the plan "Cosmote internet on the go me karta". You can read my post for more info, as well as the recent post of DeBilbao regarding this plan (on the previous page). This plan comes with 1. a SIM or 2. a SIM and a USB Modem. Since you already have the Huawei Wifi you won't need another modem so just go for the Started Pack with a SIM only that is much cheaper.
Re your Huawei WiFi Modem, I don't think that you will have any problem given that it supports the Cosmote/European 3G bands. I have the previous model of the same modem, and it worked without any issues with all three Greek operators. I guess yours is the one on the right, mine is on the left:
http://farm5.static.flickr.com/4112/...cd6ec14f_o.jpg
Some tips that applied on my Huawei WiFi and probably apply on yours too:
1. Make sure that the modem is not SIM-locked, i.e., it's unlocked for all the networks. Also make sure that you can add new APNs setting. To do both, my modem needed first unlocking and then a firmware update.
2. The modem can be configured while wired (through USB in a laptop) or wirelessly (through a web configuration menu via a phone/tablet/laptop browser). The second method is much more versatile since you will be able to change or check any settings of the modem without any cables.
3. Check if you can make new settings for a Cosmote/Vodafone/Wind connection (APNs etc) while you are connected to the modem's web configuration wirelessly. It's better to save those connection settings before you come to Greece. Then, the only thing you'll need is to insert the Cosmote SIM, connect to the modem through your phone/tablet and choose the Cosmote pre-configured connection.
Hope this helps. If you need any additional information don't hesitate to ask.