two dozen eggs = 24 eggs exactly
several dozen eggs - could be 36/48/50 etc. eggs
You can't say two dozen(s) of eggs or several dozen(s) of eggs here.
Again, many hundred eggs is not the same as many hundreds of eggs .
As for #2, see above.
dozens/hundreds/thousands/millions etc. of something is a lot of something, and can't be preceded with a number of quantity.
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