TYPO3 Core

To load the TypoScript for an arbitrary page in FE you can do something like:
$tmpl = GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\TypoScript\\TemplateService');
$tmpl->start($rootLine);

Then $tmpl->setup will contain the TypoScript-setup for that page. But condition-matching (PIDinRootLine, ...) still works against the original page (using $GLOBALS['TSFE']) and not the page/rootline of our new TemplateService-instance.

Temporary workaround:
Overwrite parts of the global TSFE (of course backup them first) before calling $tmpl->start().
$GLOBALS['TSFE']->id = $returnPid;
$GLOBALS['TSFE']->tmpl->rootLine = $rootLine;