Fiat currencies are based on nothing but trust and the desire to produce. If everyone decides to live on chickens, beans and veggies from their yard tomorrow, one couldn't use their money to go out and buy a car because no one is making them.
The bigger question for UBI isn't where the money will come from, it's what effects will it have on who does what for whom.
Anyway, as it is, people do want to hunt, gather and produce so money is an efficient way exchange value. BTW, I encourage anyone to read David Graeber's "Debt: The First 5,000 Years". He posits that money (and debt) has always been around as a means for friends and family to exchange favors and things of value. He argues that money has been around from the beginning and that it did NOT come after barter. Fascinating stuff.
Are people already getting the things UBI would pay for? I'll argue they are, only not paid for directly by people from their checking accounts. Instead of UI, people get it from forms of social assistance.
Let's say they're not. So everyone gets $2,000 and goes out to spend it. Say on a car. Would it employ more people in the auto industry? Are there people available to work? If there are people with nothing to do, and UBI would pay them to do something, then it would be a win-win. Indeed, we're always printing money to match the increase in population!
What if people get the $2,000, want cars, but no one else wants to work, then we have inflation of course, because there is more demand than supply.
Unfortunately, human behavior and other economic factors are so complex we can't really predict what would happen if we start UBI. Remember, many predicted the end of the World after the creation of the Federal Reserve and Social Security and Medicaid etc...
UBI is not a thing we consider how to pay for. We already have a progressive tax system, which re-allocates wealth by printing money for some and burning it for others. We already have a property based society where families have wealth that basically gives descendents tax- and work-free lives while others have to work to generate money. In that sense, taxing rich is NOT about taking their money, it making them have to work more and those without wealth, having to work less. Not arguing it is fair or whatever, just that it has nothing to do with money itself.
The real question is, do wealthy people put their money in libraries which I would say is good say where UBI would force more wealth into individual drug use. Complex questions.
In the end, UBI is the distribution of national wealth--what we want our common wealth to be used for. I believe automation is the greatest threat to people sitting around finding ways to be irritating. Something must be done.
So I agree with your ultimate fixes. But I don't worry about how will pay for UBI. I also wrote about UBI last night in a more humorous angle.