Does anyone have a source for the suggestion that:
If all Israel respected the Sabbath if only one single day, the Messiah would come immediately, for it is written: “Today if you were to listen to his voice”?
I found this but it doesn't exactly say the logic you listed in the question
Shemot Rabba 25:121; Yerushalmi, Ta’anit 1:10 “Though I have set a limit to ‘the end,’ that it will happen in its time regardless of whether they will do teshuvah or not… the scion of David (Mashiach) will come if they keep just one Shabbat, because the Shabbat is equivalent to all the mitzvot.”
The gemoro in shabbos 118A says if two shabbosim are kept the geula will come.
I found the Gemorrah to be Shabbos 118B not 118A, Rashi in Bamidbar, at the end of Shelach says that the Jews had already kept one Shabbos, before the story of the Mekoshesh Etzim - wood gatherer, so it makes sense to say that this would be meaning - should the Jews keep one more Shabbos (a total of two), the Geula would come.
I believe it's a misread of the end of Yerushalmi Taanis 1:1
ר' אחא בשם ר' תנחום בי ר' חייה אילו ישראל עושין תשובה יום א' מיד היה בן דוד בא מ"ט [תהילים צה ז] היום אם בקולו תשמעו א"ר לוי אילו היו ישראל משמרין שבת אחת כתיקנה מיד היה בן דוד בא מאי טעמא [שמות טז כה] ויאמר משה אכלוהו היום כי שבת היום לה' וגומר חד יום. ואומר [ישעי' ל טו] בשובה ונחת תושעון בשובה ונייח תתפרקון:
The Gemara says that if they would Teshuva it would come that day based on the Pasuk that says “Today if you were to listen to his voice” Then immediately afterwards it says that if they would keep one Shabbos they would be redeemed based on other Pesukim.
See also Shemos Rabbah 25:12 where the mistake also could have came from although its slightly less likely.