I love, love, love your new route! I'm so glad you're going to Salta too, it's so pretty! (I didn't mention that before as didn't want to throw another option in there! Make sure you can still get into the salt flats though (not sure if you can go en-route to Uyuni, or you might be able to do a day from Uyuni...not sure...)
In Uyuni, this is a good hostel: Hostal Quara, Santiago del estero 137 (like a little family home - and they have empanada nights - the local delicacy, yum!)
When I went to Brazil, I couldn't even say thank-you! I actually had no intention of going, I was in SA to learn Spanish, but I met a friend at Iguazu, and she persuaded me to go to Rio - which I agreed to...but 'just for 2 days'...and I stayed 5 months, lol! (I can speak Portuguese now, mind!) The places you are going to in Brazil, being close to Argentina, will more likely understand Spanish anyway - plus the people in the hostels will speak English and will help you out - Iguazu is no problem at all! You'll be fine with the language - it's unusual for 'gringos' to be able to speak Portuguese - if you want, I'd recommend the Lonely Planet Brazilian Portuguese phrasebook - really good - and it tells you how to pronounce the words phonetically (I assume the SA Spanish version is just as good) Lot's of people do travel in Brazil though without being able to speak Portuguese and they manage
From my own personal experience, I didn't find Brazil dangerous, but I appreciate it can be, but moreso in the big cities of Rio and Sao Paulo - I lived in Rio for 5 months, and was fine. It's just about taking the precautions you would normally elsewhere. Probably the biggest thing is on buses (and this is in all of South America), keep your daypack close to you - not in the rack, or on the direct floor underneath the seat in front of you. I used to rest mine on my legs/feet in front of me - a lot of daypacks get robbed (or some of their contents) on long bus trips when people nod off to sleep.
Finally, I wouldn't worry about buses in South America - the bus network is incredible, and you're not going off the 'gringo trail' in any route you're taking, so you will be fine
I'm so jealous! Boa viagens!! (Happy travels in Portuguese