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I am not in aviation, but if they added redundant sensors and a new indicator, with an accompanying change to the flight manual for how to react to said indicator, wouldn't that have run contrary to the goal of "no new training/certification required"?

I agree. That and the resulting sales impacts would seem to be the key motives to these decisions.

There are likely document retention policies that limit how long the relevant emails and text messages may be retained. There could be Word files or other documentation that captures discussions still floating around though.

Hopefully whoever is investigating this is acting fast to acquire the emails before they automatically get wiped.

From what I've read, the assessed severity of the failure of aviation systems is rated. The rating for this system was not severe enough to require redundancy based on the assigned rating. I'll update my comment if I can find a reference.