Sorry it took me so long to get back to you here. I wanted to be able to sit down and give this the respect and attention that it deserves.
First and foremost, I'm sorry to hear that you're in such a bad way. It sounds like you've been through the ringer. Sending lots of positive vibes your way and hoping things turn around for you soon. At the risk of sounding trite or cliche here, how do you eat an elephant?
One bite at a time.
It would appear as though you're eating an elephant right now. You'll need to tackle one step at a time here, but first you have to figure out what it is that you want to, and realistically can, do.
Turning my attention first to the tank situation, I agree with
@Nman wholeheartedly- some of the nicest tanks I've ever seen were "budget" systems. That being said, even "low cost" systems have a basic cost. This may not be what you want to hear, but if you are struggling to make ends meet or cover the basic expenses of even RODI cartridges and membranes, would it be wise (or realistic) to continue? This could also be a contributing factor to your water quality issues Something to noodle over...
As far as the dinos go, they are a royal PITA, but the battle is winnable. You have a UV sterilizer, so that's a big plus. How old is the bulb? Also, if you haven't checked it recently, is it still working / is the bulb on?
You said that there are issues with your test kits. What are you currently using? Do you have any one close to you geographically that you can lean on? Maybe a club member that could loan you test kits or reagents? We will need accurate readings of your nutrients to tackle this.
Manually remove / siphon as much out as possible and then do a blackout for 72 hours. Your tank inhabitants will be okay . When you turn your lights back on, only run blues. You will need to do this (light discipline) for the foreseeable future
Are you currently running carbon, or do you have the ability to run some? The die off from the dinos will be toxic and you'll want to mitigate that.
Patience and good husbandry are key, but again, the battle is winnable. It just won't happen overnight. This will be a lather, rinse, repeat situation.