1 ? It's better to look at AppGameKit Classic v1, v2 and Studio as iterative releases.
I'd argue it would be better if they were called AppGameKit 2011, 2014 and 2019... as this more accurately reflects them in relation to each other.
2 ? Yes
3 ? This is a much more difficult and involved answer.
At present because AppGameKit (Classic and Studio) is entirely designed around the OpenGL ES 1.1 Specification... this means that neither Graphics API (OpenGL 2.0 or Vulkan 1.0) are being fully supported.
While to a degree I do get why TheGameCreators took this approach of limiting Feature Support., as you never have to worry if a Platform Supports the feature you're using; the downside is that adding Vulkan Support was a bit like buying a Bugatti Veyron; only to add a limiter that provides similar performance to a Ford Fiesta.
And this is more frustrating to consider, given there are features that are Mobile Only., or iOS / Android Specific.
It just makes it feel like the decision made are entirely focused around Mobile Development, with support for Desktop (Linux/MacOS/Windows) merely being a side effect of supporting them as Development Platforms.
That's just frustrating because you look back at AppGameKit' predecessor Dark BASIC Professional., and it was MUCH more heavily focused as a Development Language, API and Platform as opposed to a Scripting Engine connected to an "OK" Middleware Engine.
Mind both usually are "On Sale"., so I'd suggest just waiting until a Steam or Holiday Sale Period and just get both for the same price you'd get one.
4 ? Yes. There are some deprecated functions, but other than that as noted in my first answer it's better to look at these as iterative releases.
In fact I'd argue TheGameCreators could better fund the project if they used a Subscription or Yearly Release approach.
5 ? That entirely depends on how comfortable you are with C++
I personally tend to use AppGameKit as a Rapid Development Environment., as the BASIC-Style in AppGameKit Script and fact I don't have to setup a Project is just easier to crack open and block out ideas.
And most of the time the limitations in the speed of AppGameKit Script are usually "Fine" for Desktop Applications... where-as for Mobile., outside of small arcade experiences; it's usually best to block out in Tier 1 then port to Tier 2 for better performance.