Is this like an official job where they pay you with a paycheck or direct deposit and send out a w2 and all that or is this more like "show up at least this many hours a day/week and I'll give you some cash" and you can get away without reporting it? Where did you find this job? Does the dude seem like a junkie?

If you're desperate and it is a way to for sure get some money and can be a stepping stone for a better position (which it sounds like it is), you might as well just tough it out and play the game as best you can until you can move on. If it's so sketchy you can't trust you'll get the hours and be paid for the hours, personally, I'd say fk that. But I'm pretty good at getting odd jobs especially during mowing or shoveling season. 15 sick days a year isn't the worst either, unless you get ridiculously sick for weeks at a time at some point or end up in the hospital for a couple weeks. One of my official jobs we couldn't take any time off without 2 weeks in advance and no sick days. If we didn't give the two week notice we would have to find somebody that had that day off to cover you, which would be difficult because we only got a weekly schedule... I never understood that, but ok.

I've also found a lot of jobs might discourage new employees because they want to kinda "test them" to see if they're willing to put up with any BS. Sorta let them know so the employee doesn't get surprised by any policies you say are red flags.

You can get a doctor's note without having an appointment a lot of the time if you just call and ask explaining you had some symptoms that didn't require an appointment but you need a note saying you're better (at least my nurse, doesn't even have to be a doc, did that for me a couple times a few years back).

If he got upset over the change in orientation, I'd say that would also be situational. If you agreed to a date and asked to change it, it could be saying to them that you might be an unreliable employee, but if you didn't set it in stone yet that's something different. And certain jobs that are hiring need employees ASAP especially if it's a small business that needs more employees than they have because someone left without notice for whatever reason, and will look for whoever can fill that position the quickest so they can get back to regular business.

It's ultimately up to you if you want to persue working there. Other jobs exist. Not saying they'll be any better, but they're there.