What is the closest Test series ever played, by average runs per wicket scored by each team?
And how many players have lost a Test in which they scored a century in each innings?

In India's win over Australia in 2001 in Kolkata, the two teams' average runs per wicket differed by just 0.004 • Hamish Blair/Getty Images
There have so far been 506 series comprising three or more Tests. The closest of all in this particular respect was the three-match Border-Gavaskar Trophy series between India and Australia in 2000-01, which included India's famous victory in Kolkata after following on, mainly thanks to the heroics of VVS Laxman and Rahul Dravid. India, who ended up winning the series 2-1, averaged 34.164 runs per wicket overall, while Australia averaged 34.160 - a difference of just four-thousandths of a run.
I believe the only man to suffer this strokeless fate was the Australian Ken "Slasher" Mackay, against India in Kanpur in 1959-60. In the first innings he was lbw padding up to the offspinner Jasu Patel, who was on his way to figures of 9 for 69 (and 14 for 124 in the match): you can see that dismissal in this clip (Mackay is the second of the two left-handers dismissed). In the second innings Mackay escaped Patel, but offered no shot to Polly Umrigar, who was also bowling offbreaks, and was out for another duck as India neared their first-ever Test victory over Australia.
Scotland's seven-run victory over Zimbabwe in Edinburgh last week was only their third victory over a Test-playing nation in a T20I. They also beat Bangladesh in The Hague in July 2012, and Ireland in Al Amerat in February 2019. (And they beat Ireland twice in 2015, before they gained Test status.) In all, Scotland have won 30 of their 67 T20Is, including seven victories over Netherlands and five against Kenya.
You're right that there were 19 lbw dismissals in the County Championship match between Durham and Essex in Chester-le-Street in May 2021. That turns out to be a new Championship record, beating 18, which had happened five times previously - all since 2010 - including a month beforehand, in Cardiff, in the match between Glamorgan and Sussex. Nine of those fell to Ollie Robinson, which equalled the first-class record for a single bowler.
Eleven men have suffered this bittersweet experience in a Test match. The first was the great England opener Herbert Sutcliffe, whose 176 and 127 were not enough to stave off defeat against Australia in Melbourne in 1924-25. The most recent instance was by Brendan Taylor, for Zimbabwe against Bangladesh in Mirpur in 2018-19.
Steven Lynch is the editor of the updated edition of Wisden on the Ashes