The problem you are having with multiple lines was caused by defining your file as only containing a single line. Here's what you need:

    :default ::= action => [values]
    :start              ::= Definitions
    :discard            ~ ws

    Definitions ::= Definition +
    Definition         ::= Alias <new line>
[download]

As for the backslash'd newlines, to treat them as horizontal space, you want something like:

    ws                  ~ <ws element> +
    <ws element> ~ [\s]
    <ws element> ~ backslash [\n]
    <backslash>          ~ '\'
[download]

I did test both of these with your multi-line example and they seem to work. The Sudoer's format is newline-sensitive -- it treats horizontal and vertical whitespace differently. You'll probably need to nail down exactly where newlines are OK according to the spec, before you're 100% compliant. Marpa has a mailing list, btw, where expert users hang out. Several of the people on the group have more experience with parsers for this kind of file than I have. You might consider joining up.

I hope this helps.