First of all the question(s) are not dumb. But none the less there are way more than one question and they should've been numbered 978-984.
The cat converter is basically improves the quality of the exhaust through a catalytic process. If there's less hydrocarbon in the exhaust it will catalyze less, but will not be adversely affected by this.
Monitoring exhaust temps is really not necessary with the EFIE. It has a built in safety factor in that it will drive the ECU into open loop before it can lean the mix to dangerous levels. But if someone were leaning their mix by several different means, and not actually adding a combustion improvement technology, this could become an issue.
Wow, I don't know what "gurus" think the sensor data is used last, but that doesn't even make sense. The amount of gas in the mix is the first datum you would need to know to then determine if more or less was needed. The EFIE reportedly works on wide band sensors, by the way, but is known not to work with AFR (Air Fuel Ratio) sensors.
You might read Tuning For Mileage
. It covers tuning the various devices, and covers the factor of the ECU compensating for changes. Basically if you cover all the bases in that article you should have the ECU at YOUR mercy, not the other way around.
Good luck with your project.