Many late-coming technologies are issues of concept. Slow-sand filters are incredibly simple, for example, but require an understanding that did not arise until the 19th century (some possible examples in Mesoamerica excepted). Gliders are not actually particularly difficult to acquire the materials or tools to make, especially in cultures that use kites, but require, again, an understanding of the principles involved to provide controlled flight (with gliders requiring precise balance of lift and drag, whereas kites primarily need only light weight and wind stability). Fire pistons are conceptually advanced and were not discovered in Europe until long after they were in Southeast Asia, despite some Southeast Asian cultures which produced and used fire pistons being literal stone-age cultures, in terms of available material tools.
Hot-air balloons, I would imagine, follow the same principle. If you lack the concepts behind the invention, the only way to discover it is by accident, which is generally unlikely. It’s why so much material development is concentrated in the modern and early modern period - we started to piece together how the world, on a fundamental level, works, beyond pure trial-and-error.
Also, hot-air balloons before plastics were often made of silk (and rubber), since most other materials are either too heavy (at least for practical purposes) or too thermally sensitive, and that’s an expensive experiment to be undertaking if you haven’t the first clue how it works.
I wouldn't even go that far. Having iron weaponry when your opponents have bronze is an advantage, yes ... but I really doubt that it's a decisive advantage. It's not like a bronze-wielding army is suddenly going to become helpless against a foe with iron weapons. If the bronze-age army has better numbers, better tactics/strategy, and/or better motivation, they may very well defeat an iron-age enemy in battle.
Still, though, they definitely are going to notice the other guys' iron weapons and figure out that iron is superior. And at that point, they're going to be highly motivated to find out how to make some of their own. So even if the bronze-age side wins, they will probably be adopting iron very soon afterward.
A few times, sure, but on a historical scale, it's a ticking time bomb. One of the advantages of iron, other than being significantly superior for usage in edged weapons (armor is a mixed case), is that it is much more plentiful and cheap, and doesn't require distant supply lines to be maintained in war time in most cases.
"Their swords are better than our swords" is bad, but not necessarily spelling doom. "Their swords are better than our swords and they can make 5x as many for the same time and cost, and they aren't reliant on their trade ships not being pirated or sunk by storms to continue making them" is a bit dire.