> How do you deorbit Venus is such a way that your inflatable balloon city comes to a gentle stop, 30 miles up, and just floats there?

Aerobraking with the balloon? A balloon the size of a city would slow down very fast in a thick atmosphere like that, then slowly descend until it reaches the stable altitude.

> Worse still, when people need to leave and return to Earth, how do you launch a rocket from the balloon city and return to orbit?

Spaceplane? Take off like a glider on a slope runway, then ignite the rocket.

> If there aren't materials to effect a repair, how long until it sinks down into the hellzone? Where are materials manufactured for repairs, and how quickly could they be manufactured and sent there?

That's a true problem. Local manufacturing would be on the surface. An elevator or cable car could make the connection to the ground facilities, if the winds aren't too strong. But the manufacturing of the materials would be extremely difficult in surface conditions.

> If a rocket is trying to land and goes off course, does it squash the balloon city like a grape?

Nope. See first answer.

> Saying that holes can be patched is well and good, but there are some truly catastrophic failure modes that don't seem entirely unlikely.

Possibly.

But the biggest question is why go there?