Answer 1

It's doubtable but u can suggest them by using feedback option or by mailing them at [e-mail]

Answer 2

You could send these suggestions to Sololearn by email or feedback.

Answer 3

They would need to implement a local server within the app to communicate with the hardware (otherwise no playground option) through the usb port.
Not all mobile devices would support such feature
That is impractical.

Answer 4

I think you can program arduino with c++ and raspberry pi with python. There are quite a few programs out there for free and forums if you have a specific project but also instructables has classes on both and even if that doesn't work I'm sure that the online pages of these products have guides/tutorials.