Is it a safe to say that Glenn, Kent & Valencia Pride are VERY close in flavor and the only reason to have more than one of these trees is for seasonal reasons or tree characteristics?
They're a little different from one another. Glenn has the best bouquet of the 3 but also the mildest flavor typically. Valencia Pride's flavor ranges considerably season to season and can go from being quite good with high brix one year to mediocre the next. Kent is the most consistent and has a more upfront peachy flavor compared to Valencia Pride, whose flavor notes are more drawn out with slightly less acid component. Kent and Glenn are closer to each other in flavor when Glenn is experiencing drier conditions than normal.
So while they can be lumped together as falling under the 'traditional Florida mango flavor', they are different enough that I generally wouldn't describe them as *very* close in flavor.
In your opinion next to Edward what is the best mango with standard traditional "Haden type" flavor. If a noob came to you saying they wanted to plant a Haden what's the next closest variety would you would try to talk them into getting instead?
If they wanted something that tasted identical to Haden with superior disease resistance and yield, I'd recommend Van Dyke. Van Dyke tastes exactly like Haden. You would have difficulty distinguishing between them in a taste test at the same stage of ripeness.
If their location was not too far in the interior and disease not as much a concern, Bailey's Marvel is quite good. Its often compared to Haden, but it's superior due to higher brix and lack of fiber.
Cogshall is another good alternative.