I am stuck up with the following text : Dem SPIEGEL ist es gelungen, Gespräche mit Menschen zu führen, die über längere Zeiträume im Land unterwegs waren.
Q1 > The verb "gelingen" does not involve movement or change of state. Still, why should it go with "sein" and not with "haben"
Q2 > "The Spiegel" (News paper, masculine) is the subject of the verb. Still why it is in dative, and not in nominative. There must be some rule governing this. Can anybody throw some light please ?
Q1 - There's no real logical reason for this, the best I can say it that the "verbs of movement/change of state" rule isn't so much a rule as it is a guideline. It's a pretty good guideline in that it holds true more often than it doesn't, but there are exceptions nonetheless, gelingen being one.
Q2 - "gelingen" is virtually always used impersonally with "es" (think "es gibt"), and so "der Spiegel" is actually the dative object of the verb, and "es" is the (impersonal) subject. It's a peculiar construction and there aren't many verbs that behave like this, so it is really just important to learn this example more or less in isolation. Therefore, "es ist dem Spiegel gelungen" means "Der Spiegel succeeded" or somewhat literally (not completely literal, otherwise it would make absolutely no sense) "it was succeeded by der Spiegel). In the sample sentence you gave, "dem Spiegel" was placed first for emphasis.
Your example are 3 sentences put together in one.
Dem Spiegel ist es gelungen. ( https://dict.leo.org/englisch-deutsch/gelingen ) "Der Spiegel" is a weekly german magazine. It is a name so you use the dativ and conjugate. "Die Zeit" (the newspaper) -> "Der Zeit ist es gelungen."
Er (the magazin Spiegel) hat Gespräche mit Menschen geführt.
(Gespräche mit) Menschen, die über längere Zeit im Land unterwegs waren.
So "gelingen" belongs to the first sentence it doesn't refer to a movement (im Land unterwegs is in the third sentence.)
I hope this helps a little bit.