First of all, congratulations on finishing VCE!
You are able to take language units in the free elective spaces, you don't have to be doing a double degree with Arts. If you want to take it first year though, you would have to do some reorganising of the
unit structure - same thing goes for if you want to major in it too (don't take my word for it though, I believe you'll be able to talk to appropriate staff if you do end up attending Monash).
If you received confirmation that you are eligible for the Monash Guarantee through email, yes! As far as I know, if you're eligible and meet the listed guarantee ATAR (and meet prerequisites), you are guaranteed a spot. I think the indicative ATARS in the physical book may be referring to the non-guarantee ATAR requirements.