Nice list, Hoodjem, and fun to read through and think about.
I agree you with about 1995 and 1999: like you said, Agassi and Sampras can be described as strong #2’s in those years. I think Borg can safely be described as a strong #2 in 1976, not because there’s no case for him as #1: but Borg had not beaten Connors in three years and their U.S. Open meeting was in many ways the match that decided #1; it showed that Connors was still dominant over Borg.
Wilander in 1983 does have important wins over McEnroe at the French and Australian, though I would still describe him as a strong #2 for the year because McEnroe won the January 1984 Masters and beat Wilander decisively there.
Sports Illustrated thought Wilander had a strong case but this is what they wrote after the Masters:
The Mac attack also effectively halted all debate about who is, or was, No. 1 in the world for 1983. It is, or was, McEnroe.
…. The issue became more quarrelsome than usual this year because, for the first time since 1976, the Grand Slam events had four different winners—Yannick Noah at the French Open, McEnroe at Wimbledon, Jimmy Connors at the U.S. Open and Wilander at the Australian Open. In addition, a fifth player, Lendl, led the money list with more than $1.6 million in earnings. Still, many observers maintained that the obvious choice for No. 1, regardless of the outcome of the Masters, was the man who had won the sport's most prestigious title and had finished first on the ATP computer. And that was McEnroe.
But wait a minute. Let's go to the mat for Mats. Wilander finished 1983 with a 79-10 match record (to McEnroe's 60-11) and a 15-5 mark against the rest of the Masters' 12-man field (to McEnroe's 9-6), and he was 7-4 against the other top four players in the world (to Mac's 5-5). The 19-year-old Wilander also was the only player to win tournaments on four different surfaces. All told, he won nine events, to McEnroe's six, and his Davis Cup record was superior to Mac's, 8-0 vs. 2-2. Moreover, last year McEnroe played Wilander mano a mano three times on three surfaces in three fairly significant tournaments—the French, the ATP Championships in Cincinnati and the Australian. Wilander won all three times.
Lendl had said McEnroe was #1 for the year even before he met him in the Masters final. After McEnroe won the final, Bud Collins told him there could be no doubt left about who was #1 for ’83.
In Bud’s book he still says McEnroe settled the issue at the January Masters, in what he describes as the 13th month of the season. He's ambiguous about it, though, because he says that for “breadth of accomplishment” Wilander was “Player of the Year” (then he goes on to list everything Mats did in the calendar year).
Yet for all that, I still can’t put Wilander as #1 because he was not regarded as rising to #1 in the world until 1988. Until then there were still questions about how hard he wanted to work to become #1, questions about his confidence. In all that there was the assumption that he had not yet made it to #1.
In '88, I never heard Wilander's rise to the top described as a return to #1; it was described as a breakthrough, a long time in coming.
So I see '76 and '83 as similar. Each year had 4 different Slam winners, and rather than settling the tie with smaller day-to-day wins, I prefer to settle it by looking at the big matches at the big events -- by looking at who was "ready" physically and mentally to take the top place from his rivals.