Just because there was an unexpected change doesn't mean you didn't do everything "right". You are both educated, and you found each other. There will always be good and bad, its just how you deal with it together that is important.
It's a big deal for you, I know. I've gone through it, most of my friends have gone through it at some point too. Take a bit of time to adjust, try not to freak out, worry, think of worst case. When you start worrying about money or anything else, try to focus on a future that is beautiful instead. Sound silly, but it really does help.
I would hold off on any purchases, including RODI, unless it was pretty much free. If you are low on cash then cutting your water changes in half and buying RODI from the LFS is cheapest short term. And the suggestion about talking with the LFS manager is a good one. Or go to another LFS if that doesn't pan out. Then you and Mike can celebrate his new job with a 6-stage system
If you need salt or other tank emergencies then just PM me and I'll order you some from BRS or amazon, deliver to your door. A $25 bag of salt certainly doesn't break the bank for me. I appreciate that you are trying to do the best for the animals you committed to keep the best you can.
Not necessarily, but fish love it and it tends to make the the N/P rise slower than the concentrated pellet foods. (so you have fewer water changes)
They will be fine though. I fed brine for a year before I learned there was better. I think of it as giving your fish McDonalds instead of a nice home cooked meal.
If this becomes long term then get some pellets to feed along with the brine.