I argue the studies don't
have to answer these questions, because we already have the (uncomfortable) answers.
> 1. Does it cause inflation
On its own, yes, it would create inflation if rolled out globally. However, this is why serious advocates of UBI also point out that we need policies that ensure it isn't turned into a wealth pump into the upper/ruling classes. This entails things like rent, margin, and price controls on necessities like food, shelter, healthcare, education, and transportation, which also requires substantial public investment in those areas to deter or discourage privatization of those necessities. In other words, it means pissing off every landlord, upscale grocer, private hospital, utility company, telecom, private university, rideshare company, rail company, and Taxi owner for the sake of the public good.
> 2. What is the impact on employment/productivity
Generally speaking, humans want to do something productive with their time - it's the definition of productivity that varies from person to person. Those who think solely in economic output will say that negative-profitability employment isn't of value, even if that output has knock-on positive effects in the economy (such as the disparaging compensation for teachers and first responders despite the immense value they create in the long run). It's also why people sneer at the homeless or panhandlers as being "problematic" and "undeserving" of assistance for failing to "pull themselves up [by their bootstraps]."
In reality, taking into account the whole picture, these sorts of individuals could (and studies show, often do) contribute to society better if they had support structures in place that prioritized long-term stability instead of quick KPIs for grant money. It could even be argued that UBI enabling humans to stay home and not work is itself a profitable exercise, since it removes friction from systems that those individuals might create by being forced to exist and interact with modern society and its lack of safeguards. Not only that, but the productivity gains could likely increase as we can finally eliminate "bullshit jobs" and give people the safety nets needed to start their own businesses instead - more restaurants, service experts, inventors, researchers, teachers, and other jobs that are high risk/low reward in the current system, wouldn't be in an economy with UBI and associated policies to provide for necessities and essentials.
> 3. Can we afford to tax ourselves to pay ourselves in a way that makes sense?
We can, but this is arguably the hardest opposition to surmount because it fundamentally means higher taxes on everyone, and that's a bitter medicine nobody wants to swallow. Countries with successful social safety nets have higher tax rates, but also higher quality of life as a result. Unfortunately, the current technocrats and their sycophants (not to mention their industrial predecessors and their decedents) have managed to convince a plurality of powerful people that the world would be perfect if everyone just worked harder and became billionaires themselves so we'd have no more poor people and thus no need for taxation in principle - a fantasy so high and lofty it makes the MCU look plausibly accurate.
It would mean building a society of high taxes, rigid policies, and a massive reduction in (or outright elimination of) loopholes for wealth preservation, especially across generations. It'd mean punishing hoarding of wealth, as that would (rightly) signal exploitation rather than success. These are things most people simply aren't ready for, because they still believe themselves to be one lottery ticket, one inheritance, one startup, one IPO, or one crypto boom away from being billionaires themselves, and don't want to accept that the outcome of the 99% is to work for the rest of their lives before dying in destitution of some form under the current system - a depressing reality to be sure, but all the more reason to change it.
So there you have it. If all you look at is the current system as-is and shoehorning UBI into it, then obviously you have a slam dunk case for why it's a bad thing. UBI advocates like myself, however, acknowledge that it's merely one component in a larger transformation of society itself towards more equitable outcomes, one where colorful pieces of paper are inherently devalued for necessities like food, shelter, healthcare, education, and transportation, while increased in value for actual luxuries. It's an inversion of our present society for the betterment of all, and that's why it's incompatible with the questions you raise above (which seek to preserve a broken status quo).