For device compatiblity, an unofficial tip is that if there isn't a native Hubitat driver and you can't find anything in the Hubitat Community forum, try searching the SmartThings Community forum for a "device type handler" (or DTH or device handler), the equivalent of "drivers" on that platform. Most DTHs will port quite easily from ST to Hubitat (you can do most of it yourself with find-and-replace but I'm sure people here would love to help if you don't feel comfortable doing so), and if it's a Z-Wave or ZigBee device that works on ST, it's likely to work here, too. 
Hubitat does indeed not support copying the secure Z-Wave key, though besides Home Assistant (where I needed to configure it myself) I'm not sure how many other platforms do. I never got the hang of PLEG on Vera, but Hubitat has both Rule Machine and the ability to write custom apps in Groovy, plus an array of native apps and commnunity apps here, which will save you from having to write so much logic yourself in the first place (ST and Hubitat are the only platforms I've used where the concept of apps, sort of "templates" for automations where you plug in devices and other parameters, eixsts--it seems like most make you write all your automations from nothing).
Looks like we both said "no" to HomeSeer for the same reason. 
For the thermostat, you can create something custom in Rule Manager based on how you want to handle switching between "comfort" and "away," or you can hope that Bruce will introduce a "thermostat manager"-type app in a future firmware update as has been hinted: Thermostat scheduler app like Mode Manager. I also have a custom app that turns my thermostat down (in the winter) when I'm away, in my case based entirely on motion sesnors and when they stop seeing movement for a specified period of time (not quite what you want and putting it back on the equivalent of "comfort" is a manual operation, but it's something): [RELEASE] Turn Thermosat Up/Down When Away (based on motion sensors). Other people likely have similar setups. It's definitely something an app (or rule) can handle in any case!
