Elliott's biggest accomplishments in track & field were in the 1,500m - Olympic Silver Medal and Commonwealth Gold. He was a world class athlete in the distance and held a personal record of 3:32.69 as well as 3:49.20 for the Mile.
So your remark that "Elliott was an 800m specialist" is false. Rudisha and Kipketer are examples of 800m specialists, do you reckon to see the difference?
Elliott just as Ovett and Coe which we are discussing extensively here was world class in both distances - 800m and 1,500m, and that is another fact of reality that apparently you fail to admit.
In fact as a career he was the closest comparison to Ovett because both started very young setting records at the 800m and progressively ran the 1,500m more often and the 800m less often.
His best year was probably 1990 were he set his 800m (1:42.97), 1000m (2:16.30) and 1,500m (3:32.69) personal records all a few months from each other.
And you are wrong when you say a runner can't be use as evidence to estimate what another runner can do. You absolutely can especially when one runner is clearly faster over multiple distances; if a runner is faster at 400m AND 1,000m it has to be faster over 800m too. Using your own argument:
"Elliott was a 48.2 400m runner, a 1:42.97 800m, a 2:16.3 1000m and a 3:32.69 1,500m, maybe he could have run these a bit faster but not by much, after all he raced extensively in his career, chasing wins and records, if he could run significantly faster he would have done it.
Likewise, Ovett was a 47.5 400m runner, a 2:15.9 1000m and a 3:30.77 1,500m, therefore it is simply impossible by basic track & field knowledge that he couldn't have run within the 1:42 range, as he had to be faster than Elliott at 800m with these PRs".
You won't admit because you've gone too far with your argument, but yes, it is perfectly reasonable that Ovett could have run ~1:42.5 for the 800m and you're just wrong when you say there's no evidence for that claim. It is just that you can't see it, or you don't have enough knowledge in the elite level of the sport to look at the stats of his races and make the rational conjecture that 1:42.5 was perfectly reasonable.