Based off of the table below (strictly for Jordan Poole) can I draw the conclusion that because his OnCourt +/- is way better than his On-Off +/- that he isn’t the first person to blame as to why the Wizards are so bad? If that is incorrect, what conclusions can I draw?

No - the On-Off is showing that the team has been 37.2 per 100 worse with him on the court than with him off the court. Using his OnCourt number there, you can figure out that the Wizards have been +14.1 per 100 when Poole is not on the court.

Other than some rounding being used differently between the two pages (as this is showing a +14.0 with him off and -37.1 on-off for him), you can verify that here: https://www.basketball-reference.com/players/p/poolejo01/on-off/2024

A couple shoutouts to guys who had standout volume/efficiency for the era:

2003-04 Peja Stojakovic. At the time, for the standard 3pt line (not shortened like '95-'97), his 240 3s made was the most in a season and 43.3% was barely behind the Ray Allen season mentioned in the OP for highest 3P% with 200+ made. And he put up that career-high efficiency while being thrust into the #1 scorer role due to Webber’s injury.

1988-89 Dale Ellis. He wasn’t the only player to surpass the previous 3pm record that year, but it took Michael Adams nearly 130 more attempts to make only 4 more. Ellis shot 47.8% from downtown on just over 4 attempts per game while putting up a career high 27.5ppg.

And just a little fun fact: In 1983-84, Darrell Griffith made 91 3s for the Jazz. No other team in the league made 80+.