I'm also wondering whether I should be using Flash and Adobe AIR at all. I know that the Flash browser plugin is essentially doomed, especially considering recent events, but do you think that Flash and AIR for mobile devices will last for another two or three years at least?
It has some overhead to packages, but if you've already got it written then go for it.
The existing tools are not going to vanish, and assuming you're building a standalone app with AIR, projects you build will continue to function. Building them as standalone apps adds around 10MB I believe, but that is easier than hoping the person has AIR installed and available.
I don't know how to implement in-app purchases or adverts
Both are things that should be designed early into the project. When you design them late and try to shoe-horn them in, they tend to feel like money-extraction rather than natural expansions to the game.
You write that you want to do 2 free content packs and 43 paid content packs. While I think the numbers need adjustment (you want to get them hooked on the fun and demanding more, are 2 packs enough?) the task ultimately isn't that hard. You can request from the system a receipt of what they've bought or a list of entitlements (products the user is entitled to). It is common to have several unlocking systems since you likely want to be on Apple's store, Google's store, Amazon's store, and possibly more. Abstract it away inside a query function to see if a particular module is unlocked.
I'm uncertain how to go about the release.
That is a more broad business question. You likely will need some sort of announcements and advertising, especially if you don't want to be lost in the shuffle.
Apple and Google both are growing by over 1000 app releases every day. Without marketing you will almost certainly be lost in the crowd. Note that for big games, the marketing budget is usually larger than the development budget.