Since you're pulling an HR and answering inside the quote, making it a little more difficult to respond point by point (a bitch move, I might add), I'll just respond directly.
First off, your timeline is waaaaaay off.
-For starters, it hasn't been 20 years since the series started. It's been ~2 (Ichigo was 15 when it started, he's 17 now).
-Second, Isshin gave up his powers 20 years before the series started (as it had only been a couple months from the time the manga started to when he killed Grand Fisher). This is shown when Urahara approached and asked "How does it feel to take the form of a shinigami that you abandoned 20 years ago?"
-Third, stop looking at the wiki, which states "MORE THAN 10 years ago" in reference to Isshin encountering one of the mock arrancar. Manga clearly states it's been a few decades
(meaning more than 1 decade). Since he hasn't been a shinigami in 20 years, it's extremely difficult for him to have encountered/fought/got beat by a mock arrancar in the past 10 years. Especially when the scene we've seen at the end of this chapter shows the first time he met Masaki, which was ~20 years before the start of the series.
So, to sum it up. Timeline = 20 years before the series (5 years before Ichigo is born (not all couples have kids immediately after getting married)) Isshin loses his powers through unknown means (assumed to be Final Getsuga Tensho, not confirmed) after meeting/being saved by Masaki.
-14 years later (Ichigo is 9) Masaki is killed by Grand Fisher.
-6 years after that, Isshin regains his shinigami form through unknown means and kills Grand Fisher.
-Almost 2 years later (18 months since Aizen's defeat and Ichigo regaining his power), Isshin and Ichigo have the long awaited flashback of Isshin's time as a Shinigami.
Next point, potential captain spot.
-10th division is confirmed as a "no". The captain before Toshiro was killed in combat about 110 years ago (confirmed in the manga). Databook confirmed that the spot was vacant until Toshiro got it.
-11th Division is confirmed as a "no". 110 years ago it was confirmed to be the 10th Kenpachi.
Zaraki is the 11th Kenpachi (confirmed by the databooks). The 9th Kenpachi was killed some time before the Turn Back the Pendulum arc, dismissing Isshin since he was still a Shinigami as of 20 years before the series started.
-1st, 4th, 8th, 12th, and 13th are a "no". 1st, 4th, 8th, and 13th divisions have had the same captains for the past 1000 years. 12th has had Maryuri as Captain since Urahara left (he was left in charge of the research division, should anything have happened to Urahara, and with no lieutenant he took captain).
-3rd, 5th, and 9th have had Gin, Aizen, and Tosen since after 100 years ago. Those spots are eliminated.
-Leaves 7th division, since we don't know when Komamura took the spot after Love was exiled.