Thanks for your questions. Let me try and answer.
1. Yes, you could improvise a backup like this with practice over time. The goal of these backup tabs is to give you basic ideas you can use in lots of songs. So I recommend taking your favorite backup licks from this tab and trying to apply them to other songs you know.
2. No, this backup would be difficult to play and sing at the same time. I’d probably recommend simplifying it quite a bit if you were going to sing the song as well. This type of backup works best if someone else is singing (which is why you don’t see a lot of singing banjo players).
3. It’s hard to predict how long it will take for you to make progress as everyone is different. If you’re working on new things for 1 hr a day, you’ll definitely see improvement. The most important thing is having fun which is the biggest predictor of whether of not you’ll keep playing.
4. You can play any type of music on the banjo. You could look up the chords to a Greek folk song and find them on your banjo or learn the melody play the notes. I’d start with the chords and try doing some basic rolls, after that see if you can find the melody notes.
5. No there’s no set pattern for finding the melody in a bluegrass song because every arrangement is different. Some stay closer to the melody and some venture further away from the melody. The melody is usually found in the chord tones (the notes that make up the chord you’re playing). Knowing those notes (G,B,D for a G chord) is the best way to start finding the melody to a song.
Good luck.
Mike