Hans:

"How do you explain the 'lack' of Atlantis having been destroyed"

The Account is not clear whether only the capital city "sank" in floods and quakes, or whether the whole large island "sank" in quakes. It also only says what happened then, it doesn't say what happened after and since, so we can't assume that the city and island must have remained and still be "sunken". Also the exact nature of the "sinking" is not so literally certain because different parts of the text and different translations say different things like some say it sank while others don't but say it was swallowed up and/or other descriptions. So people may be taking it too overliterally that the island "sank/sunk".
I give strong evidences that the Tiahuanaco city and area was in floods and quakes etc, and if the whole island sank I also give evidences for how South America could have appeared to have "sunk" or been swallowed up or other descriptions in either crust displacement and/or continental shift in my chapter on the sinking of Atlantis [www.allempires.com] .

"a city that remotely looks like what Plato described"

I give alot of evidences that Tiahuanaco had concentric rings, and similar size palace/temple, and cisterns, and other things Atlantis city had in this chapter: [www.allempires.com] . Bear in mind that Tiahuanaco was flooded and shaken in quakes and apparently covered in lava so it won't look exactly the same now.

"no sign of a robust naval capacity in SA to allow them to invade the Mediterranean"

Firstly the Account implies that they invaded Europe and Libya and Asia slowly over a certain amount of time, and then they collected their forces for a final assault on Egypt and Greece.
Secondly in my paper/book chapter on the Distance of Atlantis I give some evidences for seafaring boats in South America, and for greater ancient seafaring in general. Some examples include:
An ocean going ship of exotic design found while digging mines on Peruvian coasts.
The legend of Naymlap tells of the arrival of a great fleet of balsa-reed boats (Chimu/Mochica, refs Sitchin, Pears).
10 ships Paraiba inscription (refs Kolosimo)?
There are evidences that Tiahaunaco was once a great sea port. "Tiwanaku has all the features of a harbour". "Sea ports in the Andes" (Velikovsky).
Peruvians had "sea-worthy rafts" (ref Readers Digest 'Vanished Civilisations').
Chimu (Chan chan) pottery figure of "reed boat" titled "life on the ocean wave" (ref Readers Digest 'Vanished Civilisations')?

"11,000+ years ago, besides no Athens at that time or Egypt for that matter?"

The fact that Athens and Egypt/Sais were not around then confirms along with some other evidences that the date in the Account is not literally really 9000 yrs before Solon or "9500s" bc.
No one has definitely decoded the "9000" yrs date but some people agree that the evidences are that it is not literally correct. The EGyptians are well known for such exagerated code dates like 36525 yrs, 17000 yrs & 15000 yrs & 11340 yrs in Herodotus.
One theory which is backed up by quotes from CLassical sources is that the Egyptians called months years.
Another is that it is 900 yrs, which might match Moeris 900 yrs in Herodotus. Sais 800(0) yrs might match Salatis/Salitis/Saites of the Hyksos, and so 12th dyn Moeris 900 yrs & 15th dyn Saites 800 yrs could then complement each other.
Atlantis' true date ranges in sources from 9500s bc to 1400s bc to 19th/20th dynasty to Persian period.
The date for Tiahuanaco has a similar range in sources.
Peruvian culture is also one of the few oldest cultures in the world.
I haven't yet finished rewriting the chapter on Atlantis' Date matching Tiahuanaco.