Ta! to @Hot 'n' High & @A0283 for the insightful comments.
I'm not sufficiently into the things of FAA's AC 25.1309-1 nor the subsequent "Arsenal" draft to comment, beyond the fact that since the 900ER (with the plug) was introduced in 2007 extending to almost 500 currently flying around, so far no plug has separated on any of them. From that point of view, the likelihood of a "fatal accident" is therefore to be considered negligible. And another thing we can conclude is that, at least in Boeing's perception, the opening of the plug (15° on its straps/retainers) is not a "door" issue, while the plug's removal is. So, it depends if its "structural" or a "door".
While I agree that on the occasion of the second rivet repair on 714AL any informal and/or fleeting involvement of the Boeing "door" team cannot be fully excluded, I presume that in particular Boeing had a strong incentive to just "open" the plug and not to remove it (in their "logic", opening doesn't need to be recorded, removing does). Remember that on 12 Sept, 714AL had already gone through its structural integrity and pressurisation test. Removing the plug would in all likelihood necessitate a new test, which would take time. When a week later, the plug was opened on 18 Sept for the second rivet repair, 714AL was already delayed and naturally parked on the platform outside the assembly hall. This might explain why, in stead of removing the plug, Boeing went through extraordinary efforts to "open" it far beyond its standard 15° by removing its connections with the straps and putting it resting at least at 45° on an external (improvised?) support structure on the outside platform, as can be clearly seen behind the cabin window in a photo sourced from Spirit and reproduced as "Figure" 15 in the NTSB's preliminary report.
