Nice post overall. The UBI would work just the way current government subsidies do to keep the poor where they are, as you more or less stated. I also agree that price inflation would be almost immediate with the implementation of a UBI. Another great point is that the UBI is not free, but is wealth redistribution--either through direct taxation of actual producers or a reduction of the currency's purchasing power for everyone (if they simply 'print' the UBI money), which of course leads to price inflation.
There is one issue I have with automation that no one ever talks about: "What happens when the lights go out?" What happens if the cost of fuel gets too high to fuel the machines? Yes, there will be new technologies for energy, but will they be destroyed like Tesla's Tower when there's no money to be made from it? What happens when human beings get so far away from knowing how to take care of themselves without the machines that they cannot survive without them? It is not difficult to imagine, using the very idea of exponential technological advancement. Extrapolate the basic life skills lost by the majority of youth of today over the next couple of generations. Beyond the boomers, most of us have not seen true hardships in our lifetimes--like wars requiring drafts of our loved ones, the Great Depression, Hurricane Katrina, etc, where the world around you simply falls apart and you don't know what to do. We have a false belief that things could only possibly get bigger and better from here on out. It is not a sense of doom I'm pushing. It is a realistic observation of the ebbs and flows of societies throughout the history of mankind. Civilizations rise and fall. Catastrophe happens. Humans must never lose the ability to take care of themselves without having to google it. Nature cannot always be conquered.