Is it possible to simplify even more the next CFG?
S -> SS | (S) | ε
I've removed all the other null and unit productions, but I believe it is possible to simplify it even more.
How about
S -> (S)S | ε
Does that cover all the bits of the original?
S -> (S) is covered by the second S being ε
S -> SS is either two ε, check; or contains at least one pair of parens, check
Yes, that looks about right. You can choose whether you like (S)S or S(S) better, should work the same.
Note: not very authoritative, I have only a vague idea what I'm talking about