First question, according to the Eclipse Documentation 'Ctrl + /' maps to the source action Comment.
That doesn't look like auto suggest to me ?
That said, the GtkSourceView version of Anjuta appears to not support Comment / Uncomment for some unspecified reason according to a Mailing List post, apparently the Scintilla version does however. There was talk of working around this using a Python plugin but I don't think anything eventuated from that discussion.
Regarding code completion in Anjuta , it should automatically provide symbol completion , and call tips along with class members if it has indexed your symbols properly, this should be done as you are typing (code-ahead).
Regarding your If not clause, see the following relevant StackOverflow Posts
and various others that were closed for being duplicates.