Does wearing an “isdal” (a long, loose overgarment) constitute a “libas shuhra” (garment of fame/ostentation) if it is uncommon in local customs but is gradually spreading, especially since the prevailing attire is the “tarhah” (headscarf) which does not cover as much as the isdal?

I am not aware of the specific description of the isdal in question. However, if it meets the conditions of the shar'i hijab, which are: covering the entire body except for what has been exempted, not being an adornment in itself, being thick and not transparent, being loose and not tight, not being perfumed or scented, not resembling the clothing of men or disbelieving women, and not being a garment of fame (libas shuhra), then there is no harm in wearing it. If it violates these conditions, then covering oneself with something else is preferable, especially if it is considered unusual in your country, as it would then fall under the forbidden category of libas shuhra.