Too many uses of “either”?
Solution 1:
If we format the example like this:
For either [of the situations previously described]
(1) “predefined”
or
(2) “user”,
shows either (i) predefined‐only
or
(ii) user‐defined‐only types
that are either (a) valid in (for predefined) [the current language or later]
or
(b) that were defined in (for user‐defined) the current language or later.
You will see that there are not too many uses of either. You should omit none. The general guideline is that you use "either" as many times as is necessary to make your meaning clear.