English question, please help?

The piano ones are correct.
However the second question is a little more difficult.
With the first sentence 'Their life is good' it can be correct, but only when meaning the things together, like you can say "John and Marie are married with four children, their life is good" But you can't say "John is an accountant and Marie is a lawyer, their life is good" in that situation the second sentence is correct and you would say "John is an accountant and Marie is a lawyer, their lives are good"

Summarized, the first sentence can be used if you are referring to their lives as a whole.
The second is used if their lives are separate.

He is currently doing Piano at Level 10 or
He is currently undertaking Level 10 Piano

2. Depends on which context you are using
"Their life is good" would apply to a tight-knit group like a family or close circle of friends
"Their lives are good" would apply to a broader range of people in a similar situation, like a whole class of students who enjoy campus life, (rather than a close knit group with a similar focus)

I play piano at level 3
I play piano on the third level
on level 3 is most common
life and lives are understood as the same

Its-He is level 10 at piano

and the second one is correct :)
<3

level __ IN piano. not AT.

And their LIVES are good is the better sentence. yes because its plural. their + LIFE doesn't work, their implies more than one usually.