It seems like it lets employers off the hook for paying a living wage; in this sense, it’s like food stamps in the US: we’re socializing the costs of underpaying people

What about people who do not have employers? What about people who have disabilities that prevent them from working or building up an employment history that would let them work? What about the elderly? What about children?

Not everyone in society is an employee.

Making employers pay for basic human needs, like Healthcare in the US, means that if you lose your job and get sick you what, can just go die? In this situation it's not employers underpaying employees, it's the government acting as a buffer in the system that disconnects basic human needs from your employment status.

If it isn’t paid for by increasing taxes on the top earners, this would be even more the case, since everyone but the wealthy is pooling the cost?

Everyone but the wealthy is already pooling the cost of mass homeless and addiction crises, and people not having the social support and safety nets that they need to be able to meaningfully improve their lives.

I’m also confused as to how it isn’t inflationary: without either price controls on necessary goods and/or public options for housing, wouldn’t this result in companies raising the floor on prices and eating up the benefits of UBI?

Giving consumers money is not inflationary. Full stop.

Companies raising prices and price gouging is inflationary, and that does not happen in the face of consumers having more money, that happens in the face of inelastic demand or markets that are broken in other ways. A truly competitive market will still keep prices low even if consumers are wealthier since they are competing and undercutting the firm next to them to get your business.

Broken non competitive markets that are dominated by massive corporations will price gouge and do their best to suck up excess consumer profits, but that has nothing to do with UBI and by that logic why give consumers money at all or ever try and raise their standard of living? Two things need to happen, we need to empower regulators and competition laws to prevent corporate consolidation that causes inflation and sucks up excess money in the system, and we need to provide people with enough money to achieve a basic standard of living. Both need to happen and both cannot and will not happen simultaneously with one stroke of a pen so you may as well start working on one of them if not both.

Housing is slightly trickier and requires different solutions, since you don't want to encourage limitless growth into nature (hence greenbelts), which constrains supply of inherently in demand resource (and might suggest that maybe capitalism, a system based on limitless growth, isn't the best system of resource allocation when it comes to housing) but again, this is already an issue with corporations and landlords increasingly profiting off of consumers in the current market, giving consumers more money doesn't change that.

And this is the part that worries me, as someone who knows people on ODSP (Ontario, Canada’s disability-payments system): what’s to stop some jackass right-wing politician from freezing, means-testing or cutting UBI when they want to “balance the budget”?

That's a problem sure, but that's a problem right now with all other social programs. ODSP has effectively been frozen since Doug Ford got into power / has slid backwards since those payments are not tied to inflation or cost of living.