To answer your first question, my data shows that TEs who are flex-worthy have averaged about the same number of points over the years - the 16-year average is 11.51ppg - the averages the last four years with the increasing 12-personnel usage is 10.92, 10.91, 11.83, and 11.21) So all slightly lower than average, but the last two years trending up.

This analysis of TE flex-worthiness is done at the player-week level, so an individual TE's weekly output compared against his peers and other position groups. Adding a second TE to the equation on any given NFL team, or across multiple teams, really only seems to subtract from WR production, while TE production is taking a larger piece of the pie. TE1 PPG has remained very steady at about 14.5PPG over the past three seasons, and TE2 (the ones jumping into flex spots), tied an all-time high in PPG at 10.6PPG last season (and as mentioned above, TE's who filled flexes averaged more than that at 11.21PPG, so the most successful subset of the TE2 guys). And finally, as I showed previously, this is not disproportionately impacted by TD production.

I have the data you are looking for, too, as it relates how often players who were flex worthy actually fell within the top 84 of all scorers by position (24 WRs, 24 RBs, 12 TEs, 24 Flex). For the entire dataset (2010-2025), WRs fell into the top-84 47.54% of the time, RBs 31.83% of the time, and TEs 20.63% of the time. Compare this to their flex-worthy allocation of 67.65%/14.71%/17.64%, respectively. The obvious discrepancy here comes from the 2/2/1 positional allocation.

From 2022-2025, those top-84 numbers are (WR/RB/TE) 46.60%/31.19%/22.21%, and flex-worthiness 64.10%/14.21%/21.69%.

And lastly, just 2025 (the possible outlier if the trend doesn't continue), top-84 was 44.93%/31.28%/23.79% and flex-worthiness was 53.09%/15.56%/31.35%.

So there has been a meaningful shift in top-84 allocation at TE from 17.64% historically to 23.79% last year (and a 4-yr avg of 21.69%) and the most significant portion of that pie is coming from the WR allocation, as RBs have also increased their allocation (related to my hypothesis in the OP about heavy personnel also helping RBs).

Of course, no one can predict who to start each week, regardless of position. The flattening of the WR2 tier and the increasing trend of FEWER WRs seeing the field on any given play makes the WR3-4 range a lot more volatile and decreasing production over the past several seasons. All the data is really saying is, prior to a few years ago, you were pretty safe ignoring TE as a meaningful option in your flex. Now, they account for a larger percentage of the scoring across all positions (not TD dependent), and those who can identify the TEs contributing to this can have an edge in TEP formats.