Begin of pure grammar, but not finish yet

1.For some reason it is a difference for ANTLR if we have '}_;' (_ denotes whitespace) or '};'. In the second case it is recognized as one token although all white space is skipped anyway. Is this a problem with ANTLR or our grammar?

2.For the grammar rule defined in line 58 we get the following error message:
error(211): E:\Studium\setlxCompiler\setlx_pure_grammar.g:58:5: [fatal] rule factor has non-LL(*) decision due to recursive rule invocations reachable from alts 2,3.  Resolve by left-factoring or using syntactic predicates or using backtrack=true option.
We considered left factoring, however are not sure how to apply it in this case.

3.Are we allowed to use backtracking?

4.Are we supposed to use the latest version ANTLR 4 or should we use ANTLR 3.x?

5.Are we supposed to support set definitions like x := {1,3..21} ?