Are the two mentioned stories true, and is it permissible to use them as an argument for the Sunnah of specifying the number of prayers upon the Prophet (peace and blessings be upon him) as 1,000 or 10?

We do not know the authenticity of attributing these stories to their narrators. They are for admonition and solace, not for argumentation. The virtue of sending blessings upon the Prophet, peace and blessings be upon him, is great, but adherence to a specific number has not been reported, and persistently doing so and considering it part of the Sharia may fall under the category of additive innovations (bid‘ah idafiyyah). We have already clarified that the deceased benefits from the reward of good deeds dedicated to them.