But I see an opportunity for loco's to engage pay to fly, non typed MPL holders to act as 'pilot observers'.
You know, there are no MPL holders without a typerating, as the MPL is only issued after LIFUS (Line Flying Under Supervision) has been successfully completed.
Why a rogue FA? With only two in the front for the vast majority of the time, what is to stop a rogue FO or capt doing a here's Johnny?
Nothing, never has, never will. However, it is a lot easier to get into the FA position than in the FO/CPT position for those with malicious intent. Just apply to an open position (and there are always some with turnover rates of more than 50% per year) and you're there.