Sivan 5717.
Regarding what you wrote about the Sefer Torah—it is surprising that there is any doubt, since you yourself write about the established precedent to correct from a Hei to an Alef. So you should do. See also Sefer Mishnat Avraham, section 32, and in my letter printed in Sefer Otzar HaPoskim at the end of part one.
Summary
The Rebbe teaches that when there is an established halachic precedent for correcting a Sefer Torah, one should follow it without hesitation and consult relevant sources for further support.