Is it valid to say: "Peace be upon you, and upon the righteous servants of Allah" in the Tashahhud instead of: "Peace be upon us, and upon the righteous servants of Allah," and is it obligatory to make up the prayers in which this صيغة (formula) was recited?

The obligatory part in the Tashahhud is to say: "Peace be upon us and upon the righteous servants of Allah." Saying "Peace be upon you" in this context is a clear grammatical error (lahn jali) that invalidates the Tashahhud. You are not required to repeat your previous prayers because many scholars do not consider the Tashahhud obligatory. Acting upon a less preferred opinion (qaul marjooh) after the act has occurred, and the difficulty of rectifying it, is something some scholars have made lenient. Moreover, Shaykh al-Islam chooses the view that whoever omits a condition of prayer out of ignorance is not obligated to repeat it.