Most of the places you've mentioned do have 3G coverage from one or all three mobile operators but as I said the quality of the signal/bandwidth strongly depends on the exact location. EDGE coverage is not very ubiquitous in Greece, since the operators prefer to upgrade to 3G/HSPA networks.
However, all those places have GPRS coverage almost everywhere and from all three operators. For email emails, GPRS is just fine as long as you don't need to download big attachments. Also for browsing with your iPhone, I recommend using the Opera Mini browser when in GPRS, since it offers up to 90% less data consumption and therefore it is x5-7 times faster than Safari.
It's been many years since I visited the places you plan to go (except Lefkada where I was last summer) but most of them are popular tourist destinations so I suppose that you won't have any major problems. In Lefkada, I had 3G in most popular places/beaches and in the center of the town there are numerous cafes and taverns that offer free WiFi.
Regarding the WiFi modem/router of ZTE or Huawei, there's no need to purchase any of those since you've got an iPhone. The new iOS 4.3 which will be available on 11 March, will have a new
Personal Hotspot feature where you can connect up to three WiFi enabled devices. Just upgrade your iPhone in a few days and test the Hotspot with your notebook, tablet or other WiFi phones.
edit: Regarding the activation time, I just got a confirmation that in most cases is still instant. In a few cases can take up to two hours, depending on when your registration will be filed on the central database of the operator. Just make sure when you register, that the agent at the store enters your data directly to its computer (which is connected to the central database) and not on a piece paper.
Regarding the available data plans and how to activate them, I'm currently browsing through the four Greek operators and I will post an overview here later today or tomorrow.