You don’t need to change the downstream at the same time. I think the downstream mainly measures catalyst efficiency. And the up stream is air fuel?

One thing you definitely shouldn’t cheap out on is electronics, sensors, and engine internals. But the tensioner, sure

That’s what’s meant for. Changing a flat in a no so ideal situation.

Do it when the exhaust is hot. I’ve had good luck using an impact on the o2 socket (if it’ll fit where you need it)

Leaking injector probably. And the code. Possibly so much gas went into the oil that it diluted it

Sounds like a loose timing chain

$226 for drive belt maintenance? Sounds like they’re hard up for work. They bill will be a fraction if you do it yourself. And it’s all very low on the difficulty scale.