Replacing catalytic converters on '02+ models
The primary catalytic converter isn't integrated into the exhaust manifold on my '00 model. In the '00 Helms manual, page 9-3, it shows there are gaskets on each side of the primary converter. The existence of a gasket indicates a separation of 2 parts, not an integration. I'd have to believe that Honda did likewise for the following model years as well. I can't see any reason why a good exhaust shop couldn't figure out how to weld in a cheap aftermarket converter and relocate the O2 sensors to get the job done. This looks to be simple customization.
As for the Nox converter: If that converter were to fail, and it was my '02+ Insight, I wouldn't spend $1K+ to replace it. I'd go and have a straight pipe or small resonator welded in place of the Nox converter, remove the wiring going to it, and buy an '00-'01 manual transmission ECU from a junkyard off of car-part.com for $125, and swap that in, thus eliminating the potential existence of a Nox check engine light code ever appearing. I'm assuming that this Nox sensor is the only ECU sensor addition that is different between the '02+ models and '00-'01 models. If there were other changes, which one can verify by comparing wiring diagrams, then this could complicate things from being a straight ECU swap. The replacement ECU would also need to be made compatible with the ignition keys.
Thus an '02+ Insight owner with MT can deal with a situation of 2 bad converters at a small fraction of $2,000. One should be able to have all the above done for under $400.