Standalone PGE is STILL receive updates, as the work slowed due some lack of the time, look into GitHub repo and you will see lot of changes are in work. SMBX2 only uses PGE as DevKit part, and later, when PGE Engine will be completed, it will completely based on PGE (will be SMBX3) and no more will use legacy engine and LunaLua, most of API will be ported to PGE Engine directly.
Bagu wrote:1) Can you use PGE to make a sellable game (i.e. on Steam)?
You can make the config pack from scratch, it's totally unneeded to follow any existing config packs, as when you creating a new game from scratch, you don't need to rely on custom media as you are completely free to declare everything in config pack directly. The only purpose of custom media is modding allowing and ability non-vendor (anyone except of you and your team) users make own levels, episodes and add-ons for your existing game without of direct hacking of config pack.
Bagu wrote:2) Where is the best place to start making a standalone game with PGE?
Use absolutely any config pack as example (but don't use media from it, only basics of config pack creation).
As first experience, create the config pack that will contain only one element of each type, and then extend it with any other elements.
Here is the full specification of config pack setup you can use for you: https://wohlsoft.ru/pgewiki/Game_Configuration_Pack_References
And also, if you gonna to use some extras of audio engine: https://wohlsoft.ru/pgewiki/SDL_Mixer_X#Path_arguments
Some stuff related to PGE Engine scripting system (which is not so completed and will receive massive updates): https://wohlsoft.ru/pgewiki/Category:PGE-Lua_API
PGE Engine will be completely re-licensed into MIT and you will be able to use it's modified versions without of opening your source code parts you have embedded into PGE Engine assembly. However, if you are producing config pack, levels, episodes, and media, just by using current PGE Engine assembly with no modifications (you can take same assembly for every platform: Linux, Windows and macOS), you are free to do anything. If you will modify code, you will need to publish source parts you have changed.