Huh? Where does this preclude his name to be Seresh? I see how it *could* be Teresh, but nowhere does it say that it can't be Seresh; and it's a Persian name so who really knows, unless you mean that they would've spelled it with a Samach.
Sounding more and more like the מדה הי"ד שהתורה נדרשת בה.
Are you losing the sight of the forest for the trees?
Just to point out how your argument doesn't even begin to make sense, see the following possuk: וְגָלֻ֣ת הַֽחֵל־הַ֠זֶּ֠ה לִבְנֵ֨י יִשְׂרָאֵ֤ל אֲשֶֽׁר־כְּנַעֲנִים֙ עַד־צָ֣רְפַ֔ת וְגָלֻ֥ת יְרוּשָׁלַ֖͏ִם אֲשֶׁ֣ר בִּסְפָרַ֑ד יִֽרְשׁ֕וּ אֵ֖ת עָרֵ֥י הַנֶּֽגֶב׃
Would you be arguing about the spelling or diacritics of צרפת and ספרד based on how it is spelled in français or español (or castellano)?