Your sentence looks perfectly grammatical to me, although it doesn't necessarily convey all of points 1–5, but I assume you know that.
If there's anything to fix, it would be that
You should put a comma after "initially" because you have two full sentences joined by a coordinating conjunction, and
"he was skeptical initially and he had not believed" is a bit redundant.
There's multiple ways you could fix #2, so I'll just give a few options.
One possibility is to replace the "and" with a dash or a colon, which you'd use to provide additional information about what exactly the manager was skeptical about.
(1) The manager said he was skeptical initially – he had not believed his colleague that had tried it in Prague.
Depending on the surrounding sentences, that option may still be redundant. So another possibility would be to delete part of the redundant phrasing.
(2) The manager said he was initially skeptical of his colleague that had tried it in Prague [but believes him now].
(3) The manager said he had not believed his colleague that had tried it in Prague [at the time, but he does now].
Note that in (2) we flipflop "skeptical" and "initially" in order to get closer proximity between "skeptical" and "of his colleague." That improves flow but isn't strictly necessary.
In both (2) and (3), the bracketed words indicate the implied remainder of the sentence, which may or may not be necessary to add, again depending on the other surrounding sentences.